I love it when male singers sing the opening or ending themes: especially when the voice actors themselves sing them, it really brings out the atmosphere of the work, conveys the characters' emotions, and leaves a lasting impression. Like the character songs we often hear.
Female singers can also sing, for example, the female protagonist can sing herself (voicing the player).
Sometimes non-voice actors are hired to sing, and if they sing well, they might even release a music collection.