Sump pump size and reliabilty for crawlspace
If you have any fear of high water table of flooding you always need a backup on your sump pump. There are some options to safely knowing your sump pump system is always working such as sensors that send updates to your phone, however having the peace of mind that a backup pump instills is nice. If you are going to theoretically rely on your backup someday when the main pump fails, be sure to buy a backup that pumps a reasonable amount of water. Someone more experienced will need to add information about the sock.