Why do I have to have a muzzle?

Always having a muzzle with you is a legal obligation, it is not necessary to wear it.
It is extremely rare, almost impossible, that you need to use it but in case of emergencye.g. for an injured dog that needs to be blindfolded or transported, is essential for your and his safety.

Also if you don't have it with you For example, you will not be able to use any means of public transport.