The Problem is sediment filled with bacteria and calcium called BIOFILM which settles in the pipes, pumps, heater, corners.
The same process happens with a vase of flowers on the table, in a couple of days you’ll see a slimy substance, it probably sounds familiar.
An even better example is your teeth just feel your teeth after a while they also build up biofilm, that is one of the reasons that you brush them !

So the question arises why don’t we “brush” the tubing of the Spa ? Chlorine or Bromine can’t do the job alone.

If we don’t clean the pipes and allow it to fill up with biofilm there are consequences, chlorine or bromine can’t work properly,
bacteria and calcium build up and is protected by the biofilm and gets harder to get rid of.

All of this happens in the pipes which you don’t see which is, other then contrary to popular belief, NOT convenient.