I think the goal of these efforts is to provide material that gets picked up and spread by others, amplifying their message. They help to normalize an environment of divisiveness and extremism.
As intelligence services have always done, are doing now, and will do until the end of time. I do find this whole "The Russkies are Coming!" narrative bizarre; everything that's been totaled up so far keeps amounting to a pisshole in the snow compared to domestic election spending or outragism peddled by home-grown media.
The difference is perception and tangibility. We can see and count these tweets...and to some degree quantify their effect (e.g. Retweets). And this is a much greater bang for the buck with no feet on the ground.