There is already a good answer: It provides an additional quick visual indicator in case the passenger bought the ticket only after entering the vehicle and spotting the conductor.

But let's add some more context.

Ticket controls do not usually pay for themselves with fines. They are paid by getting more people to buy tickets. The goal of ticket controls is not to catch passengers who cheat, it's to encourage passengers to buy valid tickets.

There are plenty of ways to circumvent the timer, starting with "My phone just crashed, the reboot will be done in a minute", and ending with software that creates a forged ticket. But I speculate that the app creators speculated that the timer reminds potential cheaters that purchase time is relevant. And because clocks exist, purchase time stays relevant even if one of the multiple mechanisms that show purchase time is defeated with a forged ticket (forged tickets were suggested in comments to other answers). That encourages those people to buy valid tickets.

Also relevant: being caught with a forged ticket can be way more inconvenient than being caught without a ticket.

There is already a good answer: It provides an additional quick visual indicator in case the passenger bought the ticket only after entering the vehicle and spotting the conductor.

But let's add some more context.

Ticket controls do not usually pay for themselves with fines. Ticket controls are paid for by getting more people to buy tickets. The goal of ticket controls is not to catch passengers who cheat, it's to encourage passengers to buy valid tickets.

There are plenty of ways to circumvent the timer, starting with "My phone just crashed, the reboot will be done in a minute", and ending with software that creates a forged ticket. But I speculate that the app creators speculated that the timer reminds potential cheaters that purchase time is relevant. That would encourage those people to buy valid tickets.

To address comments to the other answers, which raised the legitimate concern of forged tickets defeating the timer: Purchase time stays relevant even if one of multiple mechanisms that show purchase time is defeated. And being caught with a forged ticket can be way more inconvenient than being caught without a ticket.