On a viola on its C string one can play notes at lower pitches inaccessible to a violin, as the violin´s voice only starts at the G a "fifth" above the C string that is on the viola. Hence a viola can produce C C# D D# E  F and F# which are all in a range beneath the lowest pitch of a violin.

Viola music is generally written using the alto clef, but in exceptional circumstances it uses the treble clef (temporarily) for high pitched passages. Otherwise music publishers would have to waste more paper and cut down more trees because of the extra blank space that would appear on music between the staves if viola music was regularly printed in the treble clef. (Not sure this is the real reason.)

I tried transfering the Jacky Tar score from treble clef for violin to alto clef for viola (take a blank piece of music manuscript paper and copy the violin score by moving every note in the treble clef version down by one unit). I do not regard the outcome as a success. I did it because I could not find the Jacky Tar in a viola version online (free).

You can mostly play violin music on a viola "as is". But if you do so, you will find there is no need for the viola to have the C string, as you would never use it if you confined yourself to violin scores. But on a viola you will not be able to play some of the double stops that violinists can play by bowing the violin´s A and E strings simultaneously.

Some violinists are suspected to be rather jealous of the viola´s C string. And the viola can produce double stops on the C and G strings, which a violin cannot.

You might also ponder whether cello music can be played on a viola ? Cello music uses the bass clef.