and i see many wallet give for 1 user more than 1 address (HD wallet)
Yes. You should use a new address for EVERY transaction.
An address is like an invoice number. It allows you to identify who is paying you, when they paid you, and what they are paying for.
You wouldn't re-use an invoice number, why would you re-use an address? Then if you receive two similar payments you wouldn't be able to determine who they came from or what they were for.
Excuse me for my query here, but if it applies to know whom sent us the payment for what and if an address actually works as an invoice as well as we should use a new address every time for new transaction, don't you think such rule should be applicable in the case of signature campaigns too?
(My query might sound irrelevant, but I am concerned about this rule that one can't change an address during the whole campaign).
It is convenient for signature campaign managers to just keep paying the same address over and over, and that does work, but it's not necessarily the most "correct" way to use addresses. I'm sure any campaign manager would change the address they'd pay you at if you needed them to, they probably just say you can't because if not they would have a lot more work managing the users' constantly changing addresses.