In pagan religions what exactly is meant by "invoking" a deity? Isn't it more respectful to just do devotions?

I'm perhaps more animist than pagan, I don't really know any rituals. But I wanted to get some books on deities, and some of them talk about how to "invoke" the deities. I'm having trouble understanding the concept of "invoking" a deity in worship. My instinct is that it seems disrespectful to make demands. It seems like it would be more respectful just to worship deities, rather than asking them to do stuff like Christians do when they pray. Isn't invoking kind of demanding, or am I misundersanding what the word means? I really need some pagans to explan this concept better to me. Why do you invoke the deities instead of just doing devotions do them?