It's local, meaning no shipping needed.
Posting an ad is easy and doesn't require a pay account.
Users can (and are expected to!) remove crap ads by flagging them, and reporting major abusers to staff. They don't have to put up with scammers and spammers.
Categories are easy to see and select from.
It has a wide range of functions, from buying/selling to real estate to jobs and gigs to resumes to community posts and even personals.
It's NOT a bandwidth hog. The site is bare-bones with no obnoxious ads. Because users moderate via flagging and helping other users, instead of paid staff, the site expenses are kept very low. That means no bandwidth-hogging loud animated ads and it's very easy to access even with a slow modem and old computer.
Finally, when someone does have problems, there are Help forums available that are free (other users man them), very fast, and very effective. Whether it's an issue with getting an ad to show, an ad that got flagged, or getting a phone verified, help is there. Fast, free, and accurate.
Overall, that's made it a very, very popular site.