You'll want them when you see an opponent that uses the things, trust me. They'll spot mines within a sight range (pretty sure it's lower than their full sight), and without other orders overriding them, the sweeper will do a bit of work to remove them. Pretty sure invisible mines become safe and won't blow up while the sweeper can see them, but Wehrmacht S-Mines and demo charges can still be set off by walking through them and the enemy player's input respectively.
S-mines dont have any special properties if in sight and walked on. If your getting shot at by a handheld rocket launcher tank or armored car, S mines have a big hit box, they will get hit and go off.