If the run of coax is very short to tv2 it would be wise to put it in line. The tv2 output puts out more power than the standard channel 3/4 for tv1 on older boxes. Where the tv was usually within feet of the receiver. It put out more power so that if tv2 is say for example 100 feet away you don't have a fuzzy picture. If however your tv2 is real close to receiver you could burn out the front end in some cheap tv's.
I read it in the manual. It says that it helps with interference from others UHF signals. I tried it on my 622 receiver and it worked great. It may not be the case in every situation.