In otome games, if there's a character you like, you'll usually play through all their routes. If you find the world-building and storyline interesting, you'll play through all the routes for every character—that way, you'll get a good understanding of the true ending.
With BL games, I usually play through all of them. However, if I lose interest in a character after reaching their "Good End," I won't bother with their other routes.
That said, if I'm feeling a bit obsessive, I'll usually play through everything just to collect the CGs—even if I don't pay much attention to the story.