Clearing drain

[Yod12]

I have a tar paper line from the house to septic; it's probably broken somewhere and I have to dismantle a deck before I can access it. So for now, I figured I'd snake the drain from the tub which is closest. I was planning on going through the tub drain, but I see everywhere plumbers taking the overflow drain apart and going through that. What is the reason for doing this?

 

[Snoonyb]

Google; Tub waste and overflow, and remember, the shortest distance between 2 points, is a strait line.