What is the oil tank in front of the rad for in a john deere 3650 for?

It is a reservoir for hydraulic oil for the main hydraulic pump. Its purpose is to supply oil to the main hydraulic pump during start-up to prevent cavitation. Cavitation is caused by "vacuum" created by the main pump when it is not receiving any charge oil from the transmission pump. Without this, damage and wear will occur to the main pump due to lack of lubrication, and in time will give you weak hydraulics and then require either a complete replacement or complete overhaul of the main pump, to the tune of roughly $1000-$2000. Hope this helps!