Hi @AudioGuy welcome to the ASF community.
@sptrawler has answered your questions but as I had already written a reply I’ll post it as well.
Do I still need to provide my details to Computershare?
The short answer is yes because when you buy or sell shares you must exchange the title from one holder to another & a share registry maintains these records.
If so, why?
Share registry companies like Computershare need more information to confirm your identity & to get your instructions concerning the shares you have purchased. They are the record keepers of your shares.
Who decides which Share registry holds your shares?
Share registries are appointed by the Company to maintain the records of their security owners (that’s you). A Share Registry like Computershare maintains shareholder records, disburses and reinvests dividends, transfers shares for ownership changes & handles all the correspondence for the individual companies – this is the very reason why you have been contacted by them as they are the record keeper of your shares.