I don't belive the ticket apps being discussed keep a balance, but I haven't used them myself.
For even more confusion, some contactless debit cards won't work the first time, but if you try them 5 mins later (presumably after the main office has done a test charge on the card), it then works fine.
Can't use it when you have a subscription though, so a lot of people here will still be using their plastic public transport cards for personal and work travels, including me, but it's nice to have the possibility at least.
You can travel around (scanning your contactless debit / credit card) without thinking about how much you’re paying, safe in the knowledge that the system caps your daily / weekly spend to predefined limits, essentially making it never more expensive than a daily / weekly travel card would be. The caps are automatically calculated for the zones you travel in.
It’s a very nice experience as a rider. Especially beneficial to visitors who can just scan through the gates as any Londoner would, no ticket purchase required.
As for the rationale to implement such a system — well, it’s ultimately a public service. All revenue generated is reinvested into the network, in London at least. The economic value of a fast, efficient, painless transit system that everyone uses, regardless of class or wealth, most surely outweighs any lost breakage revenue.