Spellthieves are meant to combine - or at least, make a compromise between - the magic of Magicians and the stealthiness and speed of Thieves, which may come off as either successful or not. After all, magic has powers that may grant people invisibility and the ability pull and push objects from afar... But magic also has a nasty tendency to be flashy and noisy, which is rather counterproductive to those who really need to be stealthy.
This is why Spellthieves rely on mostly Utility Magic while not in combat, and restrict non-utility spells to combat use only, mostly as a backup once silent takedowns cease being an option.
Spellthieves make no appearance in the original FRPG, as they are an innovation introduced in the game adaptations.
In the video game adaptations of the FRPG, Spellthieves - just like their names suggest - are a compromise between Magicians and Thieves. They are fast, have a somewhat wider selection of weapons than Magicians, can cast spells, but are not as good at Magic as Magicians, and are slower and weaker than regular Thieves.