Basically the difference is that the username and password can be used to authenticate with that specific site and that’s it, while a “wallet” browser extension or app can let the user sign transactions and manage value outside your site, so they don’t have to trust your site (they approve all major transactions) and your site can’t impersonate them. You may think you won’t but one of your devs could go in for example and salami-slice the balances of your members, or change all the votes in a stackoverflow election etc.