Light Infantry vs Line Infantry

[MikeV]

I've used them to screen a deployment of line infantry or artillery, also used them to force an infantry advance to slow down so it can be engaged by my artillery.

That's the classic use, in an open-field battle. After they're done skirmishing, and withdraw behind the line, they're available to protect the line's flanks (especially if the cavalry's busy chasing off the opposing cavalry). Their longer range helps them contribute fire onto the units engaged in front of the line. After the enemy foot units break, they can also chase them down (again, if the cavalry are off doing something else, or re-forming after a charge) without having to commit the line and have them lose order.

The other major use is against buildings and other field fortifications, especially when they can out-range the defenders. The line take too long, and too many casualties, to be as effective.

As ever (well, since the time of Alexander, anyway), think "Hammer and Anvil." The line, in this case, is your Anvil, and Artillery/Cavalry your Hammer. Light Infantry aren't quite peltasts, but they're useful in the auxiliary roles.