As in most languages, in English there's more than one way to say many things. The present progressive tense, when combined with something that indicates future action, is roughly the same as the simple future tense.
"I am working tomorrow" = "I will work tomorrow"
"She's flying to Hawaii this evening" = "She will fly to Hawaii this evening"
"Aren't you going to see your aunt this weekend" = "Won't you see your aunt this weekend?"
And so on. However, this is not a solid rule and there are some uses where the two are different. For example:
"What are we having for dinner?" is a question that implies that you are making dinner, whereas, "What will we have for dinner?" is a question that implies either of us could make dinner (or we could go out to eat).
"Will" can also imply a more definite situation. "I will close this deal tomorrow" is stronger than "I'm going to close this deal tomorrow."