Useful
Part of speech: adjective
Definition: Having a practical or beneficial use.
Example sentence: A life spent making mistakes is not only more honorable, but more useful than a life spent doing nothing.
Functional
Part of speech: noun
Definition: A function that takes a function as its argument; More precisely: A function y=f(x) whose argument x varies in a space of (real valued, complex valued) functions and whose value belongs to a monodimensional space. An example: the definite integration of integrable real functions in a real interval.
Part of speech: adjective
Definition: In good working order.Useful; serving a purpose, fulfilling a functionOnly for fuctional purposes, notably in architectureHaving semantics defined purely in terms of mathematical functions, without side-effects.Of a disease, such that its symptoms cannot be referred to any appreciable lesion or change of structure; opposed to organic disease, in which the organ itself is affected.
Example sentence: I'm the test kitchen manager, which means I'm in charge of sourcing all of our ingredients and kitchen equipment. I also manage the budget, help out on photo shoots, and generally coordinate all the moving parts to keep our kitchen functional.