I use Empire archers in small units for chaffing/redirecting, march blocking, screening and threatening unarmoured low T troops, especially NG Fanatics and solo wizards.
I always try to include at least a unit of Huntsmen (longbows in the 6th ed!), and often a detachment of ordinary archers. They are helpful.
As to why people might use them in big units, I'm not entirely sure. They are nimble skirmishers and can move freely. And more shooting is more. Still, they are a bit expensive for what they do.
I've seen larger units used only in lists that have some (fluff) reason not to take Pistoliers. For example the Army of Middenland/Cult of Ulric lists, or fan-made Halfling lists. Of course archers are core, whereas Pistoliers are not, so that might also be a reason.
8th ed introduced more upgrade choices in the form of command group. I remember people had ideas about stronger archer units then. I myself used a unit of nine archers and even sought to challenge some small units into close combat. But they were a nuisance at best, easily countered.
Could it be the models? The Free Company/Militia box allowed you to build ten archers, as did the new Huntsmen/Archers box. And people liked those models. So if they have 10-20 archers painted, maybe they use them in a big unit just because?
Edit: the 5th ed monopose Bret bowmen are some of the cheapest ever GW models around, and have been available in second hand ever since 1996. Head swaps can turn them into OK Empire archers. Model-wise they might be the cheapest option to build up an Empire army. I think around a quarter of my archers are these models, a nice 10-man skirmisher/shooter unit. This could also partly explain the big archer units.
-Z