What is the purpose of baptism for sola fide adherents? Sola fide says that works play no part in our salvation. For sola fide adherents, what is the purpose of baptism? Is baptism necessary?
Speaking as an adherent of the belief that salvation is by faith alone, and not of works (lest any man should boast - Ephesians 2:8-10):

Baptism is an external symbol of obedience.  It doesn't gain you anything.  It is an outward show and demonstration that symbolizes our death to our old selves, and resurrection into the new life as a Christian.  

Consider a secular comparison - graduating from school.  The graduation ceremony does not educate us.  Receiving the diploma does not educate us. Both are a sign that we have completed our education and are moving on to the next step, and both are largely ceremonial and serve as a rite of passage.

To those of us that believe in sola fide believe that (obviously) only the saving grace  of God is responsible for our salvation, and baptism no more saves us than a diploma educates us. 

As a side note, for those of us that believe in baptism by immersion, rather than sprinkling, this is a part of why we believe in this form of baptism.  In addition to it being how John the Baptist did it, we also see the going under water as symbolic of Christs death, and coming back out of the water as symbolic of His resurrection.

To those of us who hold my views, baptism is not necessary for salvation.  However, we also realize that there are plenty of opposing views.  It all hinges on whether or not you count baptism as a "work".  I've seen arguments for both sides, and all seem to have merit but for me, personally, and most of the people in my Church it comes down to one verse - Ephesians 2:10, not of works lest any man should boast.  (emphasis mine)

In giving all glory to God, we refuse to try to take any credit for our own salvation.  Even the tiniest hint of pride in the matter is not acceptable to us, and whether baptism is a "work" or not, if we feel that we have contributed to our salvation in obedience, then it is not of God alone, and the glory is not His.

Again, I know there are opposing views, but the question appears to be geared to how those with this particular view see the issue.

I would say that baptism is necessary on some level, though -not as an act of salvation, but as an outward sign that we're really "giving up our cross to follow Him".  If we can't obey such a simple thing, how on earth would we expect to claim that we live for Him?