What would you say the Viola Do ?

A bit weird question, but genuine. Considering the cello mostly plays the tonic of the chord alongside the double bass, covering the bass side of the music, though the cello shares a bit of the shimmering melodies of the violin carrying the music through, while the 2nd violin plays most times the same melodies an octave below, what would you say the viola do? what do we stand for? are we practicing hours a day for filling the hole of 3rds and 5ths? what are we looking for with the viola? do we even have a “target”?