Note: I write this answer from the perspective of somebody who has a Garmin GPS. There are others, but from all accounts (The Wirecutter, etc.), Garmin makes the best ones. The post below might sound like I'm trying to sell somebody a GPS, but I just really love mine. I have no affiliation with Garmin.
I think there's something to be said about the pure convenience of a dedicated GPS. If you don't live in a high-crime area and always leave it mounted on your dashboard, it will always be there, and you will come to always use it and depend on it, even if you don't enter a route. Would you go to the hassle of pulling out your phone, mounting it to your dashboard, plugging in the charger cable, unlocking it, and opening the maps app every single time you drive your car? I think not. You might not think this is a big deal, but it is. Usually you would only start up the maps app when you're lost or don't know how to get somewhere, but if the dedicated GPS is always there and on, you will use it for much more than guided navigation.
If your car has a switched power outlet (i.e. it turns off with the car), a GPS will turn on when you start the car and automatically go to the maps screen, without touching it. If your car doesn't have a switched outlet, like mine, it is very easy to add one (pretty much anyone could do it, all it takes is plugging in a piggy-back fuse into an existing fuse slot and connecting the other wire to a metal part to ground it).
If you buy a bit of a higher-end GPS, it will come with voice control. That is, just by saying "Voice command", you can control it hands-free. My Garmin doesn't do so well with odd-sounding street names or locations, but for the basic stuff like "Go home", "find pizza", "find walmart", etc., it works super well. I'm not sure how well Google Maps, etc. does this, but when I had my iPhone a year ago it didn't seem to have anything like that (except Siri).
GPSs also have much larger screens than most phones, which means they can show that complicated interchange that much better.
Glancing at the screen to see the name of a cross-street or your speed and the speed limit will become second nature, and you'll miss it when it's not there.
There are loads of other things my GPS does that I really, really like, that most (if any) phone apps, or even GPS's from even a couple years ago, do not do. For example:
If I'm on the freeway and there's a route set, it will tell me to "be in the second lane from the left", or "be in any of the right three lanes". This was huge for me when I first heard it. Before, it just said, "keep to the right", without telling you that there are actually three lanes that exit and you don't need to worry about trying to get to the far right lane in heavy traffic. Also, it will use very natural-sounding speech, like "turn right at the stop light", or "turn right at the tee", or "take the second left". In my experience (last year when I had an iPhone), Google Maps isn't this good, but maybe they're improving it.
"Up Ahead" is something my Garmin has. Basically, it's a little panel that sits on the side of the screen, and it shows the distance to the next gas station, restaurant, rest stop, Tim Hortons, etc. on your route, and if you tap it, will show a list of all the matching locations. I only use this when travelling long distances, but it's great to know that there's a place 10 km ahead where you can stop to empty your bladder or grab food, for example.
Predicted destinations: If I have a destination saved in my GPS and I am driving in the same general direction on the same day of the week as I last navigated to that location, my Garmin will show on the top bar "Work: 45 min, heavy traffic", or "Church: 10 min, no traffic". It's great for getting a "heads-up, there might be traffic", or even just to know how long it actually takes you. Usually, I would not actually have a route set for these locations because I know how to get there, so the voice would just be annoying.
Driving log: My GPS draws a blue line behind you as you drive. If you're in an unfamiliar location, this is great for recognizing where you are. For example, in 2015 I moved to a large US city from Canada, having never been there. That little blue line was critical for me learning my way about, because as soon as I saw that blue line on the screen, I knew where I was. In my experience, relying on automated voice directions means you never really learn the roads.
The one thing that dedicated GPS devices do not do is real-time, crowd-sourced traffic updates like Waze. For example, my Garmin receives traffic updates through FM radio, and if my phone is in the car and has a data connection, it will also pull traffic updates from that via Bluetooth, but in both cases, I think it will only do this at most once a minute, and only for major roads, which usually means just freeways.
In my experience, manually updating maps once a year is just fine. Roads don't change very fast, and updating maps is dead simple. It takes a few hours to download, but once you plug it in and press Update, it does it on its own.
I will also go against Burhan here and say that a standalone GPS is more customizable. Sure, you can get funny voices on your phone, but you can do that on the GPS too. Pretty much any language is available, so that's not an issue, and you can even change the car symbol on the screen. But more importantly, the UI is customizable. So if you don't need to see the elevation, put something else there instead.
There's more I could say, but I think you get the point: yes, a stand-alone, dedicated GPS is miles better than a phone GPS, but for most people, Google Maps is just fine.
@stanri brought up a good point in the comments: "my phone overheats on a hot day, sitting in the windscreen, especially if I'm driving around all day and charging it at the same time. I wouldn't suggest that having a phone in the sun, charging, on a long drive, is a very good idea."
