Proper drainage is very important when it comes to bathroom showers as improper drainage can cause flooding which can easily spill over to the other areas of the house, damaging floors and also can cause electrocution especially where the water comes into contact with electricity. The shower drain needs to be installed properly in order for it to function optimally as if not properly fixed there is bound to be bad smells coming from it, as a result of blockages that can be caused by hair.
Usually if the shower is not draining properly it means that there is hair clog somewhere in the shower drain line, therefore regularly checking on the strainer cover may be a good practice such that the problem is taken care of before it gets out of hand.
Traps usually help in trapping the hair that may cause all the above problems early enough and their main purpose is to prevent sewer odors from travelling up through the drain. The drains usually have some kind of trap that usually prevents foul smelling gases from coming to the sewer piping.
When installing a trap it is very important for an experienced plumber to do the work and also to advise on the best place to place it as any trap that is misplaced will not help prevent sewer odors. You can read more about this on http://www.showermania.co.uk/products.asp?cat=15 to raise your awareness on this topic further.