I do not have 50 reputation to comment, but I can click "edit" everywhere.
Of course, I do not modify other people's posts, but why can I do this?
You can do this because we allow anyone to make an attempt to make the site a better place.
Until you reach 2000 reputation your edits will be community reviewed. This means that a new user cannot simply wander on to the site and start vandalizing everything in sight. If they did then it is likely they would quickly be banned from suggesting edits as well.
Accepted edits can also earn you 2 reputation per edit, up to a 1000 total contribution to your rep, so they can be a small way to get started on the site. There are better ways to get rep, like providing great answers, but edits can be a minor filler.