Why is sorbitol used in buffers?
Generally, sorbitol is one of those 'old tricks' in the book which is thrown into cultures or buffers for several reasons. Here are a few that I have heard of:

Bacterial cultures (e.g. E.coli BL21) that are optimised to express (produce) protein, because sorbitol is thought to increase the expression of soluble protein. I have even heard of people (who work with very difficult proteins) that add to a final of 0.6M sorbitol in their culture media (note that the cultures grow very slowly).
In buffers intended for proteins, as it is thought to increase the solubility (i.e. avoid aggregation), similar to other sugar such as glycerol or osmolytes such as betaine.
Sorbitol is considered (chemically)  to be relatively inert, and many will therefore prefer to add sorbitol over e.g. glycerol or sucrose as a stabalizating agens when they work with proteins.
When you work with glycoproteins, you are generally depending on the addition of some sugar to stabilise the protein of interest - both during isolation, purification and analysis/testing.

The last point here could answer you question, as immunoglobulins are glycoproteins, you are generally depending on a high concentration of some sugar - such as sorbitol - to work with these proteins (i.e. antigens and antibodies) in e.g. immunoprecipitation.
I don't believe that sorbitol is a crowding agent - as this would perhaps facilitate aggregation, the opposite of the intended purpose. Adding crowding agents it typically done in protein crystallography with the addition of various PEGs to 'get the' proteins to form crystals. Instead, I think that sorbitol may (somehow) interfere with the proteins solvation shell - in the same way that is speculated for osmolytes (e.g. betaine or various sugars); however, this is something that I dont know much about and should be investigated.