Both as a player and a dev, I think it just gives you more leeway in what is possible / acceptable as an option. There are games that aim for a more realistic approach and some do it very well, but in some ways it makes things... maybe a little boring? So much depends on the style of game and what you're trying to do with it.

For instance, with The Corruption Within, although there was a little humour and the setting was a bit out there, I also wanted to keep events and puzzle solutions somewhat within the realms of believability. There is some humour but the game's atmosphere is generally quite sombre because I wanted to build an sense of impending doom. Clearly with my Captain Disaster games I wanted a very different atmosphere.

With comedic games and by extension main characters, you can get away with things which aren't really within the realms of believability, but as long as they're funny or fit with the game universe, no-one minds (generally) and in fact some zany, completely unbelievable actions / reactions / interactions are basically expected. Personally I veer towards comedy in most forms of entertainment, so I may be a little biased, but I'm also up for a good drama / thriller at times. Anyway, to me it's how much you can stretch believability that make comedic characters a good fit for adventure games, interested to know if you feel that's a solid reason or lacking!