Flexibility and constraint are both useful. (Constraint is one of the reasons to use a finite state machine in the first place.) An overly constrained system can't function, an overly flexible system can't be relied upon. You'll answer your question the day you encounter a problem that NoSQL is a bad fit for, and after you learn SQL it won't feel so alien or complicated. Or maybe that day will never come and you'll never feel the need to learn SQL.

For a motivating example, consider a billing or accounting system. This domain is well known, we probably don't need the ability to rapidly evolve our schema. If we violate the constraints of this system, there may be serious consequences, like spending money we don't have or billing for the service twice.

We could build this system with either an NoSQL or a SQL database, but SQL would seem to me to be the natural choice.