I'm not sure but...
Maybe survival and moving for reproduction. The live in a Pool, who can be destroyed, or their people in the Pool can be destroyed. To reach another Pool with other Yeerks they can reproduce with, they (maybe) needed to take the control of other species...
But it isn't a very good explanation ^^' I know that... Only trying...
Edit: Reading "The 7 Most Horrifying Parasites": Did you see #4? It's horrible... The most horrible to me ^^' because I DOES infect Humans!!! >_< ...and all the mosquitoes who took my blood this year in Japan... T_T