I see the Astromech and other doids are partially implemented but I dont know what the use of the droids are.

Where I can read about what the droids do?

Astromech droids with piloting skills in the cockpit prevents travel aborting if the pilot boards an internally docked entity. (Link)

Droids can be used in ground combat much like NPCs (Link) and can also be disabled and captured. (Link).

Medical droids can be used to assist in cybernetic operations. (Link)

LIN droids are used to assist in mining. (Link)

TT-8L Sentry droids report the movements of PCs, NPCs and droids through the room they are deployed in to their manager. (Read the description)

I may have missed some.

Thanks for the answer, I heard many times before about piloting droids but now I understand.
So I should buy some Astromech just in case?.

That's a decision only you can make knowing your circumstances and what you intend to do. I think there are a number of situations it can be useful to be prepared.