Some companies(eg GWR) require the ticket to be scanned for a delay compensation claim, so if the ticket has been eaten that is not possible.
Good Morning Paul,
Please accept my apologies for the delayed response.
Tickets for finished journeys are retained by the ticket barriers for various reasons. This reduces tickets being strewn across the station premises and helps us to dispose of them in environmentally friendly ways. This also helps us to reduce fraudulent use of tickets.
If you wish to keep the tickets to claim for compensation or your travel expenses, please speak to the member of staff in attendance, so they can check the ticket manually and open the gates for you.