There are a number of piers along the lakefront on Lake Erie here. Understand that the lakefront has changed drastically since we started engineering it for our use, a lot of areas that were simply marshland have been repurposed.
There are still a lot of areas here where you can walk out into the lake farther than casting distance before you hit deeper water, a pier provides access to it. You get a couple of hundred people fishing off a pier using emerald shiners or worms? You are creating a chum line?
A lot of ours have lights on or near them at night, the lights attract baitfish. Depending on the material used in the construction? They provide shelter for smaller fish and crawfish, a feeding area for larger fish.
Most of our inland lakes are man made here, flooded valleys. A road bridge or rail brige usually has the sides entering the water covered in broken limestone rip rap to prevent erosion. The rip rap provides smaller fish and crawfish with habitat, it can usually mean deeper than normal shoreline depth, and also provides a place for feeding fish to trap baitfish.