Arrival - The reason for the aliens visit. Spoilers

For this theory I'm assuming that the aliens are telling the truth and aren't hiding anything. After all they're a species who can see the future, lying would be a pointless concept to them.

The facts we are given are:

1. The aliens want to help up humanity.

2. They need our help in 3000 years.

3. Louise must use the weapon (tool) that they have given her to do so.

I think it's clear the main reason for their visit in the short term is to unite humanity into one mega country. They give out 12 pieces of a puzzle and force humans to work together to solve it. By giving Louise the language/ thought skills to see the future, allowing her to speak to China and persuade him to help unite the first and second world. I think that's all in the surface of the films plot.

So what do they need it 3000 years... Water.

The biggest clue I've got is the films score, which has a child sounding voice speak the words "no, water" on repeat.

Also there is a lot of water imagery in the film. When they arrive the landing sites are filled with mist. Which dissipates as the film goes on, showing it was related to the space ship not just usual weather for the location.

When the ships leave they disperse into mist, which could suggest that the ships are made from water, heavenly condensed, (not ice) but such compacted water that is appears black and has its own gravitational forced which we clearly see demonstrated.

The aliens live in a very moist atmosphere, the unknown gas in their chamber is breathable. So it is possible that it's water.

So that's all the physical evidence that I can see. However there is some uses of film language that suggest the same. Like I mentioned the music. As well as that, the main revelation flashback/forward when Louise leaves the ship for the last time. The one with her daughter by the lake. Firstly thy are by a lake, suggesting water. She helps her put wellies on, again suggesting a tool that helps humans manage water. The sound of water is present throughout the whole scene and on top of that its raining when we return to reality. The only time it does at the Montana site in the film. However the key part of this flashback/forward is the dialogue. Louise says "...swimming, poetry. All the amazing things you share with the world." The aliens shared their language with humans and vice versa. This is hinted at when she says "poetry" leaving swimming as the other thing mentioned. Which again Suggests that the aliens also want to share water. Obviously this has no logical implications with the events, however it's sort of a metaphor/ filmmaking clue of what is going on.

Maybe the aliens depend on water much more than we do. And possibly use it as a limited resource. So they want ours, but also don't want war, but in order to make trades they need to talk to a single world leader. As mention in the film. So their mission was to unite us now so they can trade with a more civilised United society in 3000 years.

Tl:Dr : Aliens want our water, but don't want to fight us for it so came to unite us first.