Obviously it only makes sense to show the ones that are relevant to your journey on that specific train (unless having a discussion regarding passenger rights, e.g. because of a likely missed onward connection). Nobody has any way to know that any others exist, and showing those is only likely to cause confusion.
Agreed, unless you have a connection to make that you may require assistance or advice with (e.g. if a delay occurs).
However I might only show the ones that are relevant for that specific member of staff, e.g. I may only show tickets as far as Birmingham New Street for a through journey on XC, as I know the Guard will change from that point.
If a passenger held through tickets to somewhere like St Ives on an XC service to Plymouth, a passenger showing the entire combination of tickets in Yorkshire would probably be told the platforms of their onward trains at Plymouth and even at St Erth, if their Guard was the legendary Kevin Kramer (who provides
outstanding customer service)
And he's not the only one; various others such as
@gimmea50anyday, to name just one other example, also provide details of onward connections, including platform information (even stating the next train will be the same platform face, but further along the platform, as happens at stations like Manchester Airport where Northern trains reverse 'on top' of a TPE train which has terminated at the buffer stop end of the platform)
That said, if you don't need onward travel assistance/advice, then there is no particular need to show tickets for other train services than the one you are on at the current time.
And talking of delays and split tickets, it can be very useful having an itinerary (which could be shown on a phone or printed) showing your entire journey, detailing all trains and all tickets used for the journey, which you could generate yourself but will be generated automatically if using an accredited ticket splitting website. This is particularly useful when seeking advice regarding missed connections as well as when making applications for Delay Repay.