The alternative is providing the sender address so that their system can identify who paid and for how much, which results in you having to identify yourself before each and every transaction (probably through a user account setting or some other form you have to fill out). This is an extra and cumbersome step so it creates an incentive to maintain a singular and constant BTC address of your own.

The rationale behind creating a new BTC recipient address for every transaction is that it doesn't matter who the sender is.