Monzo will be most useful when there’s money regularly coming in and out from Direct Debits and Salaries. For example, if we know how much your energy bill is, we can gauge it against all the data that we have and let you know if we think your energy company is ripping you off and help you switch to another provider! 
However - nobody can force you to set up Direct Debits on your Monzo current account, and having your salary paid in isn’t a requisite either. So essentially, if you were to only move money over from another current account that you have, and use the Monzo card for your day-to-day spending (as an example) - you’d be utilising it in the same way as a prepaid card anyway. But with the benefit of FSCS protection etc
So whilst it won’t be a prepaid card, you could still use it like one.