This reduces the chance of heterophilic antibody interference. In most antibody assays, protein is used to prevent these interactions. Heterophilic antibody interference causes both false positives and false negatives.
Also, if you just put 10mL of FBS (or FCS) in 500mL sterile 1X D-PBS, as long as you're using good sterile technique, this solution doesn't require the addition of sodium azide.
Adding a bit of protein (generally FBS or BSA) can reduce non-specific binding by your antibody as well as reduce cell clumping with some cell lines.
Souce: NovoPro 2018-03-31