I use Roll20. But only to play with my RL friends. I would never play with a random stranger.
Yet plenty of people put up ads offering to run games.
This website gives people an oportunity to get to know each other and become friends before joining a game.
As much as I have appreciated conversations with various people on here, I would not consider anyone on this website a friend. Then again, I have somewhat high standards for what I would consider "a friend." Speaking to someone on a forum simply does not tell you enough about them for them to become a friend. It may make it easier for them to
become a friend once you develop some other form of contact.
Name one? DMs are in short supply. If you are any good you find yourself beating players off with a stick. Or you charge people.
Example that might literally apply to me personally at some point: Previous game group stopped meeting because RL issues affected too many members, so the game had to fold due to insufficient players.
Other examples:
1. They have a regular group, but that group doesn't want to play a system this person wants to run, so they're starting a second game and looking for new players.
2. They just feel like running a second game, but everyone in their usual play group is already playing one and can't attend another game.
3. Recent move when their previous group only wants to do in-person games, so the DM in question has not yet built up a group.
4. DM is hoping to meet new people to bring into a larger group, and thus runs an intentional short campaign looking for copacetic players.
5. DM decides to run a system they've never run before, and looks for players who have experience with it to help make that process easier. (This is how I met my favorite 4e DM; and the game ended because the DM had a family health crisis to deal with.)
6. DM is new to DMing, they tried to build a group with friends, but (for reasons completely unrelated to their DMing skill), it turns out their friends just don't really enjoy TTRPGing as much as they do. They have to look elsewhere to build a group.
So, you asked for one. I've given you six more. Is that adequate?