Consider a site that sells digital goods for download. They need to store information that provides evidence of your physical location, such as IP address [1], in order to satisfy tax authorities that they collected the right jurisdiction's VAT or sales tax.

I doubt that you are going to explicitly ask sites to store your IP address, so how do you think that should be handled?

[1] IP address alone doesn't prove location, but it is evidence. The EU, for example, for requires for internet sales that you justify your choice of whose VAT to collect by providing two non-contradictory pieces of evidence for the location you chose. IP address can be one of those pieces. Billing address of the card used for the purchase can be another, and for most people that and IP address is enough.