Many water heaters have a check valve on the outlet, this will in fact slightly reduce pressure (you should NOT notice) but as they age minerals build up to the point where they may totaly fail to open,, In which case you are NOT going to be in hot water till you do something about it.
The one on the hot water outlet is needed if you have a single valve bypass system.. BUT.. You can replace with a quarter turn valve if you like and kiss check valve issues goodby (Provided you remember the valve spring and fall)
The one that may be on the cold water INLET.. I am still trying to figure out why they put em there..