Brass washers/nuts on exhaust manifold studs

[frmhrt]

Have seen several posts with recommendations to use brass nuts and/or brass flat washers on exhaust manifold studs.

No indications given for the reason to do this. Is this to reduce leaks or to reduce corrosion or? Any input?

Also, if I use hi strength bolts for the exhaust manifolds, shall I use flat washers/brass flat washers and why?

thx

alan