I can use many of the site's features while unregistered. I am looking for a simple explanation of what benefits a user can have from registering.
If you're not registered, you're a cookie type user. And with that, your account only stays with you so long as your cookie remains. You will also be completely unable to vote, with the sole exception of accepting answers to your questions.
If you wipe your cookies off your machine, or you want to use the site under your account from another PC, then you're effectively starting from scratch. And that means having to collect rep all over again to do the extra fun things on the site (i.e. comment, upvote, downvote).
That user will still be there, but you'll lose control over it. You might be able to restore it if you have provided a correct email address, and there are hurdles you can jump through to merge accidentally created duplicate accounts into one. But it's easier to just register.
Registering allows you to easily sign in and out from any computer you want.
If you also want to use the site through another browser, you can do that as well, but only if you're registered.
Remember the first day you were on the site, had only 1 rep and couldn't comment on a question asking for clarification, lest you did it in an answer and be thrashed by people flagging your post as spam? Registering means you don't have to go through that kerfuffle more than once.
Also, the site prevents unregistered users from deleting their own posts. Not something you'll need often (hopefully...), but still nice to have now and then. And as flagging is a type of voting too, unregistered users cannot flag for moderator attention either.