The point is to share intermediate state of your data structure without headache.
For instance, if you write an interpreter, representing variable bindings with a functional map allows to reason about scope super easily; when you change scope, you just create a new map with the new variable bindings, and when the scope ends you just get back the map at the scope beginning which is still valid.
With a stateful hashtable, you need to undo the changes you did in you scope; the value of the table when opening the scope that you want to get back to is gone forever. Now it ties you program to a certain execution order. One way to see that is that if you want to collect environment to each scope in some data structure for some reason, you need to make deep copies everywhere.
For instance, if you write an interpreter, representing variable bindings with a functional map allows to reason about scope super easily; when you change scope, you just create a new map with the new variable bindings, and when the scope ends you just get back the map at the scope beginning which is still valid.
With a stateful hashtable, you need to undo the changes you did in you scope; the value of the table when opening the scope that you want to get back to is gone forever. Now it ties you program to a certain execution order. One way to see that is that if you want to collect environment to each scope in some data structure for some reason, you need to make deep copies everywhere.