This especially comes into play for me with roach/hydra and to a lesser extent with ling/roach. I want my roaches in front of the hydras to absorb damage, but since roaches (with speed) are faster than hydras they just get separated unless I spending some time microing their movement.
With zlot/stalkers the stalkers are faster and I want them in back so I just have the stalkers on follow on the zlots and just move the zlots and they'll stick together in the right formation. I do the same thing with ling/roach even though it's not completely optimal but the speedlings will get into the fray fast enough that it's not a huge deal.