Mobile check deposit would be great for a few reasons.
Depositing Cash. Money orders are cheap < $2.00 A money order is just a check written by the store to yourself. Being able to deposit this would be great. Reloading the card directly through Green Dot costs ~$5.00 and seems to no longer be supported. https://www.attheregister.com/
Some employers only issue checks, and do not offer direct deposit.
Many refunds are only offered by check. My car insurance would only refund me on a paper check.
As there is no P2P payment system in the US that does not rely on third parties, such as Venmo, Zelle, Paypal, etc. Checks are often used for gift giving.
I know Monzo’s stance on Mobile deposit in the UK, however most if not all banks have offer some sort of Mobile Deposit in the US and have done so for almost 10 years. It is just pretty much a given here.