First, the maximum aperture is a big deal. If you work in circumstances with limited light, it makes a lot of difference, and can allow you to take shots that you otherwise couldn't.
Second - without looking into too much detail of the two specific lenses you linked to - quality. Quality of the raw glass, quality of the glass surface curvature and form, quality of the micro-positioning and micro-orientation of the single pieces inside the whole, quality of the coating(s). The consequences are a sharper image, less chromatic aberration, less reflections and less irregularities; and the wider possible aperture results in lower neede ISO and therefore less noise. Maybe also less weight, which is important for some people.
None of those are significant if you take a vaction shot in the sunshine with your kids running around - you have more than enough light, and the aberrations are invisible in 5x7 prints.
Very significant if you are trying to sell poster-size high-quality prints of a night shot for a price that allows you to live on taking pictures - in such a print, observers could see the color errors, unsharpness, and noise, if you didn't use the top lens, and nobody would pay more than 5 $ for it.