"The tuba is one of the orchestra's more spectacular-looking instruments, since it fills the arms of the player holding it. It isn't easily manageable. To play it at all one must possess good teeth and plenty of reserve wind. It is a heavier, more dignified, harder-to-move kind of trombone. It is seldom used melodically, though in recent years composers have entrusted occasional themes to its bearlike mercies, with varying results. (Ravel's tuba solo in his orchestral version of Moussorgsky's Pictures at an Exhibition" is a particularly happy example.) For the most part, however, its function is to emphasize the bass, and, as such, it does valuable service."
from What To Listen For In Music, p. 95