He is seeing Veer running around a lot for other in the mansion and for Rani, so he feels rather than wait for things, he too take some charge of some part so that Veer is a bit OK to handle to get Rani freed
Birju with Rani have won all competitions or work in the village
Clues for the accident too knowing the culprit they handled and won on that ground
So here being a Saathi for Veer is very important
Somewhere I feel it could be the trio planning too to get out RaniSa as a namesake part knowing both Vijay and Birju told someone in the family knows Rani's whereabouts
if failed somewhere Nandu may sarcastically say to Birju your dost is gone then ...spill the beans