|MadSci Network: Biochemistry|
I am not a pro when it comes to these types of methods, but hopefully this answer will help:
In general for these types of procedures, Tween helps to wash away unbound blocking agent (the BSA in the BTBS solution). In other ELISA-type procedures, or steps in this procedure, it can also prevent non-specific antibody binding (or washes away unbound antibody). Tween is a type of detergent, so its general use is to help keep moleculaes separated (prevent interactions, aggregations, etc.).
Hope this helps!
Try the links in the MadSci Library for more information on Biochemistry.