The whole point of trolling is to provoke a reaction. Trolls thrive on gaining your time and attention, so by engaging in arguments with them, you're giving them exactly what they want. They don’t care about the substance of the argument; they’re only after the attention. If they succeed in diverting your time—your most valuable resource—from something productive, they’ve won, regardless of whether you "win" the argument or not. 

 This is the original meaning of the term, (messing with someone who did not understand the meaning of a comment, or asked a dumb question), but over time people have misconstrued it's meaning into being like a smack talking keyboard warrior or whatever they make up in their head.