It's my understanding that the aliens came to our planet to teach us how to understand heptapod, their circular language and method of communicating. And they had an important reason. Being able to read or speak in heptapod means that you are able to see a person's entire timeline from start to finish.
The aliens, through the use of this foreign language, are able to change our perception of time (because the past and future essentially can be seen at the same time). Plus, in the future (like, 3,000 years in the future), the aliens are going to need mankind's help for an undetermined issue. Understanding heptapod is the tool, or "weapon," the aliens are trying to share with humanity, because they know they are going to need us later.
But what I don't understand is what kind of threat is it they can't face without our help? What could be that undetermined issue?