Most probably wanted to duplicate temperature effects and pressure effects of 2nd stage in relation to first stage- consider that in flight as first stage shuts down- momentary negative g would have some effects on valves and pressurization and fuel sloshing of 2nd stage LOX and fuel tanks. Without a detailed diagram of plumbing, valve types and sequencing one can only speculate. Liquid fueled missiles have been lost due to failures of certain valves needed to mitigate or stop flow effects of momentary negative g..