When you first start learning to play a string instrument, the fingerings are often given to you. They are either printed in the music, or else your teacher tells you exactly what fingers you should use. Then, you learn that there are often multiple options for which finger and perhaps which string you could use for a given note, and the concept of fingering starts to get a lot more complicated. So how do you know which finger you should use?
Violin and viola players often learn to shift between first and third position before they learn any other positions. I will talk more about the different positions in Part 4, but here I want to talk specifically about the decision a string player might make to play something in a different position than they normally would, even if the shift isn't the most efficient choice, for expressive purposes.
The most common reason a violin or viola player might make such a shifting decision is because they want a wider vibrato. Pinkies vary in size and flexibility, but they are smaller and less flexible than their neighbors on the same hand. A lot of violinists and violists find that they can get the best vibrato from either their second or third finger. So if vibrating a note is really important and you want that vibrato wider or maybe just want more control, you might choose to shift to a higher position so that you can use your second or third finger instead of your fourth.
Another reason you might play some notes in an unusual position is because the composer or arranger wants those notes to be played on a particular string. There is a difference in the tone of lower strings compared to higher strings, and sometimes the composer will specify a string to incorporate this tone difference in the music.
Finally, shifting is sometimes necessary for a particular technique. Ornaments requiring multiple fingers (trills, mordents, turns) might only be possible in a given position. The same is true for harmonics as well as certain double stops such as octaves.