When TVMs are deliberately not repaired when broken so as to force people to use e-tickets phased out, what are you supposed to do if you arrive at a station without having purchased a ticket beforehand, and where there are no ticket issuing facilities.
Your phone might be broken/stolen/not working, or there is no internet access to enable you to buy a ticket online.
You could say buy on board from the guard, but if the majority of stations do not have TVMs, and it is a busy train, the guard will not be able to get round and sell everyone a ticket in time.
Then there will a large queue at the destination station there will be a large queue for people to buy tickets to get through the barrier, and it will encourage short faring. (eg. there is an unpaid fares window in the Red Lounge at Birmingham New Street, but as it is at concourse level the person selling the ticket won't know, or be able to prove, whether you have come from Five Ways or London Euston)