A solid cover will help block dirt from accumulating for an easier spring cleanup. It will also completely block out light, allowing you to close the pool when you want rather than having to waiting until water temp stays below 60 degrees, or in spring, open once water temp reaches 60 degrees. If you tend to get a lot of rain over the winter (we do) and have to keep an eye on water level (pumping water out as needed), it will also prevent the chems in the pool from being diluted.

As Carl mentioned though, a mesh safety cover is the only true safety cover.