Radon loves sump pumps.
(O.K. Radon is a gas. We know that it can’t really love sump pumps…BUT sump pumps really can be a major entry point for radon gas when not properly sealed.)
While there isn’t a plumbing reason to have a lid over a sump pump, a sealed lid like the one pictured (complete with a viewport) can help keep radon gas from seeping into your home. Of course, if you have a high radon level then the gas is likely seeping through many entry points so sealing thesump pit will only alleviate a portion of the risk.
The best treatment is to give the radon a place where it is supposed to go by having a mitigation system installed and sealing any other place where the gas might want to seep in.