Nothing, but this particular alternative is pretty awful. Literally zero features besides TOTP code generation, and they don't plan to make it better. I really don't understand why this app exists.
The only people who would possibly care about it is existing Bitwarden users who want to use it to hold the code for their Bitwarden account independently from account. But they say they plan to add Bitwarden sync to it so?....
Honestly it just looks like a super lazy attempt to draw people to Bitwarden (assuming it doesn't turn into a sleazy attempt of holding codes captive with no way to get them out).