True endings are normally a thing for plot-focused games, but they're very present in pure romance stories, too.
Most of the time, a company won't go out of their way to say "this is the true ending." You just sorta know by which of them is the most complete, reveals the final mysteries of the setting, etc.
True endings can be good. They can also be bad. And also bittersweet. That doesn't really affect anything. Sometimes you have true endings being less happy than good endings, sometimes not.
It's a story and none of it actually happened, so I think it's pointless to think of the other routes as useless and whatnot. They're there for your entertainment. Same as the true one. It's only actually relevant if they're going to make a sequel.
A lot of VNs obviously have a main heroine, where most of the polish went, as well as having the deepest, most relevant routes for the plot. Those are the ones that normally have true endings.