To me, especially in arc based series, villains are mainly there to antagonize the heroes. They way the heroes react is what really makes the story. If I don't care about the heroes, I find it difficult to care about the story at all. Villains can be interesting in their own right, but I feel like that's secondary. That goes double for longer series, which swap out their villains as it goes on, but the heroes will be mostly the same cast of characters. I think the heroes basically are the story in those cases.