Of course global state is useful. All interactions with the universe (all i/o, anything with time) involve global state. So what makes DB's a smart pattern but

Of course global state is useful. All interactions with the universe (all i/o, anything with time) involve global state. So what makes DB's a smart pattern but global variables usually bad? It gets hairy whenever you have "behind your back" mutation within a single scope. Because db calls are costly, people usually batch the transaction and pass the result to mostly pure functions. If you were to do something silly like wrap a Redis get/set in a Mapping, it'd feel an awful lot like a global dict. This is why things like transactions, event sourcing, and the reactor pattern exist. You batch all i/o to the global state so you a) deal with immutable structures everywhere else b) have ACID c) have traceability.