Most major Open Source conference (for profit or not) have volunteers. These volunteers are a part of critical processes designed to help pull off the conference in the most successful way. Depending on the conference you may be called on to perform content review or just staff a registration table.
As a volunteer, you receive the conference for free. That is a substantial financial savings. Further, every conference (for profit or not) utilizes volunteers in some manner. This may be for content review, or for staffing a booth.