[crAzerk]

I've seen it in replays where people run lings into a mineral line and hit hold so the AI doesn't have them all running off to fight a single cannon or something.

Every time I try it though, it works so poorly I should've just left them to their own devices.

The more lings you have and greater the worker saturation, the better this technique becomes.

The reasoning is simple - when you have many lings and they try to FF down 'closest-target-units' (as the AI does), they will obstruct each other and prevent them from doing maximum damage. (some lings not doing damage at some times)
If you just put them on hold position they'll just hit anything that runs past them, which means optimal damage (constant damage from all lings) assuming there's decent saturation.