When something is serviceable, it's useful just the way it is. You might describe your rusty old car as perfectly serviceable if it always gets you where you need to go.
If your eccentric grandmother insists on buying you a full-length fur coat, you could argue that all you need is a serviceable winter jacket — simply one that will do its job and keep you warm. On the other hand, you might long for a beautiful pair of leather boots despite having a serviceable pair of rubber galoshes. Serviceable was spelled the same way in Old English, although it originally meant "willing to be of service," and only later came to mean "usable or useful."