Railway flyunder at Shenfield station

I grew up in Billericay, Essex and I regularly caught the train from there into London.

At Shenfield, the line from London splits - the main tracks head north towards Ipswich, and the other tracks head out east towards Southend-on-Sea:

One things has always puzzled me about this junction. There is a normal double track section on the branch line which operates in both directions, but there is also the single line flyunder which only operates in the eastbound direction - this flyunder is used far more frequently than the double line section in that direction.

Is anyone able to shed some light onto why this flyunder is there, and why it is used more than the other track?