Filling an outdoor drain with concrete to reduce standing water
Sumps typically have standing water, as Jasen mentions, part of the reason of having them is a sediment trap to prevent debris from going into the storm drain.  All that "compost" didn't end up in the storm drain.  You also likely don't want that compost entering your pipe and possibly getting a clog.  A deeper sump pit means you have to clean it out less often.  

Is there something specific you worry about?