Using a new address per transaction helps you maintain your anonymity.

If you reuse the same address and one transaction was tracked back to your identity (say for example you sent yourself funds from a KYC exchange) then ALL of your transactions (past and future) would now be connected to your ID.

This is not only a problem for you but also for anyone who transacts with you, as your identity helps to learn more about those you transact with.

While using a fresh address per transaction may not make it impossible to ID you in the above example, it does make it a lot harder. Your average person would not be able to look up one of your addresses on a blockchain explorer and discover your entire savings.

The following Bitcoin Wiki page gives a good summary of the privacy issues and illustrates working examples…