Several reasons I can think of.
Networking, these colleagues will hopefully respect you for sharing and might be an asset to you in the future.
Project and work efficiency, upskilling colleagues can make the work go more smoothly.
Promotion, you never stop learning yourself, so you should stay ahead of colleagues and step up (either there or elsewhere).
Experience, teaching skills are learnt, not inherent, being able to explain things in ways which others grasp is a valuable skill in itself.
There's plenty of others, there's also downsides as you intimate in your question.