I think there are two main issues on why to keep them and actually repair them when you get the chance.

1. Weather proofing - it prevents rain/snow etc. from getting into the engine compartment around the lights. Probably also prevents dirt and grim from getting in as well.

2. Sound Buffer - I've noticed that since mine were taken off by the repair shop, waiting on replacments but they took the originals completely off for some reason, theres been a high pitched wind noise. I'm guessing wind is getting caught in that area and creating the noise.

Another issue, that I'm not positive is an actual reason would be to buffer the hood from the lights (ie: prevent it from banging on the lights when hitting bumps), but I'm not sure on this one. Other then the above, I dont know of any reason you "have" to keep them.