Why do race cars change tires?

I'm curious to know, why do race cars change tires during a race? Is it simply to improve their grip on the track, or are there other factors at play? Do the different tire compounds play a role in determining when and why a change is necessary? And how do the teams decide when the optimal time to make a tire change is? It seems like a crucial aspect of racing strategy, and I'd love to understand the reasoning behind it.