From what I understand (and in most cases I am pretty clueless so I might be off on this) a sponsor is usually a local person who lives near WDW or DL who is willing to attend an event by proxy. The person doing the sponsoring pays for the event and tells the sponsor what pins to pick up for them. This allows people who do not live near the parks to receive the pins and gifts from an event without being there. The proxy on the other hand is able to attend an event without having to pay, they simply send the pins and gifts to the person who paid for their sponsorship.
Jeff