It's similar to people watching, or bird watching. The last two are things you do when you're an old gramps who doesn't know how to use internet or smart phones, & who's life is incredibly boring... But with loli watching, anyone of any age can enjoy. Some even find it more enjoyable than surfing the web. Like bird watching, cameras & binoculars are optional.
Guy 1: "Hey brah, wanna go to the local public pool to do some loli watching?"
Guy 2: "Sure bro, can't say no to the cuties!"