Why is it important to recognize volunteer contributions?

Summary:

Volunteers deserve the assurance that what they are doing is needed

Steve McCurley and Rick Lynch said in "Volunteer Recognition" on CASA
Nuts
and Bolts at http://www.casanet.org/program-management/volunteer-manage/retenrec.htm :

Never assume that volunteers know that they are appreciated. Recognition
of their contributions should be a part of the formal and informal
operations of the program. Volunteers who do not receive frequent feedback
and recognition begin to wonder if they're doing a good job and if anyone
cares about the work they do. This often creates an unmotivating climate
and can result in high volunteer attrition.