Because you are gaining access to a secure part of the airport (“airside”) and could be doing so under an alias. My personal opinion is anyone wanting to cross the line between landside and airside should be identifiable - ideally with a valid government ID, just in case there was an incident.

On a perhaps related note, if domestic pax can just walk through with any old name on their boarding pass, why bother making all the retail/catering staff apply for airside passes/clearances, why can’t they just turn up to work in T5 WH Smith’s as if they were on the High Street?