Unlike guitars where you use your fingernails or picks to play the strings, violins are played using a bow. Average violinists can play 4 octaves on a violin but exceptional players can play 2 octaves on a single string. Some songs cannot be played in a violin without adjustments on the pitch as these might be too low or high for the capacity of the strings.
It is also important to know how many octaves one can play in the violin as when you play a chord that it cannot sustain, the strings will snap and might only hurt the violinist. And most importantly, you can play a song smoothly on a performance if you know its chord range.