In the Avengers movie, Loki got the Tesseract from S.H.I.E.L.D., which he then used to open the portal. Since Thanos' primary objective was to obtain the Tesseract, which Loki already had and could have used to transport himself back to Thanos, what was the point of the Chitauri invasion after accomplishing that?
Thanos and Loki had an agreement in which 2 things were covered.
- Loki would retrieve and hand over the Tesseract to Thanos.
- Thanos would supply his Chitauri army to Loki, so that Loki could conquer Midgard.
The Chitauri invasion was Loki's idea, not Thanos's. It was Loki's intention to conquer Earth. Thanos was in it only for the Tesseract. This is evident from several points in the story line.
In the very beginning, the voice-over says
'The world will be his. The universe yours."
In another conversation,
LOKI - don't threaten, but until I open the doors, until your force is mine to command, you are but words.
THE OTHER - You will have your war, Asgardian. If you fail, if the Tesseract is kept from us, there will be no realm, no barren moon, no crevice where he can't find you.
This conversation highlights that "The Other" who is Thanos's handyman, clearly identifies that Thanos is in this only for the Tesseract.
Also note the usage of the words "Your war". This clearly shows that the invasion was Loki's idea, and not Thanos's.
It's similar to Thanos' deal with Ronan in Guardians of the Galaxy.
Thanos: Get me the orb and I'll destroy Xandar for you.
Once Ronan realized the orb contained an Infinity Stone, he double crossed Thanos because he could just use it to destroy Xandar himself.
He also might have been aware of Thanos' kill half policy and Ronan wanted them all dead.