What is the purpose of heat riser valve?

[1963starfire]

I own a 1963 Starfire and I am about to have someone
install new dual exhausts. I was wondering what
is the purpose or function of the heat riser valve?
Are they necessary?

Thanks

[heidigirl]

it blocks spent exhaust gases from exiting the tailpipe. its primary function is to accelerate the heating of the intake manifold, improving fuel vaporization
on a dead cold engine. if you live in a mild climate, you can get away without it, but expect driveability problems below 40 degrees on a cold engine. in a four season climate, i can think of no reason not to have it