Besides GBBC, there is another organization called Feederwatch which participates in bird count which combines its data with GBBC. The difference is that Feederwatch is more scientifically structured.
What is done with all this winter bird count data? For one thing the studies lead to more directed conservation efforts and makes us more aware of seasonal populations of birds. It helps scientists monitor:
Growth and decline of bird populations
Population distribution, including irruptions
Timing and extent of early migration
Winter climate effects on birds
Here are a couple helpful posters to help identify birds we most often see at feeders.