why not get rid of the roles at quiet stations like this one?
Presuming you mean the one featured in your original photograph, then the reason is quite simple. I can't identify the exact station from the photo but it's clearly a deep-level Central Line station, most of which require minimum numbers of staff to remain open due to fire and safety regulations in the UK as well as LU Policies and operating practices. If they do not meet these minimum staffing numbers, then they have to close. If you took this member of staff off of the Gateline / publicly facing position, they'd still need to be employed and positioned somewhere on the station for it to operate.
A notable example of this is Paddington Bakerloo, which according to minimum staffing rules should (or did, I think it may have changed recently) always have a member of staff positioned in the connecting passageway between the Bakerloo and Elizabeth Line platforms due to fire regulations. This is a position which rarely sees customer interactions as most people that are already interchanging via that passage already know where they are going and are not passing through barriers. Nevertheless, it (is/was) still a critically required position.
Plenty of metros globally have their gate lines unstaffed
Which to be fair I'd argue is unhelpful and not really providing a good enough service. I can genuinely say I've personally had experiences in two of the previously mentioned examples (Paris and New York) where I've faced issues with the barriers not accepting my ticket despite validity (especially in the days of the magnetic paper tickets in Paris which often just stopped working for no reason) and haven't been able to find a member of staff anywhere nearby resulting in me either having to cross to the complete opposite side of the station to find somebody, or in one instance, have to completely change my route and get a bus. Just because other places don't provide things doesn't mean we shouldn't too.
Whether they're on their phone or not, they're still there to provide a service (and whether it looks like it or not, are doing so) and from personal experience having previously worked on Stations as well as using the network daily myself, I've never once encountered a member of staff who has continued to completely ignore a customer when they've requested help or not sprung into action during an emergency. If they did prioritise their phone use over their active duties then I'd consider it unwise and unprofessional to be using their device (or of course at all times if currently undertaking a heavily safety-critical task like driving a train, obviously). When it comes to gateline duties (and some more active ones) there genuinely are times when there is nothing to be done right there and then other than just simply being there in case you're needed.