I believe in switching up the grips. it helps to hit different parts of the triceips.
personally, I use different ways of doing a pushdowns one arm, rope, short bar, angled bar, v bar bar and etc. if you only doing one kind of pushdowns all the time then your only develop one head of the tris. I know you can isotate a head of a muscle but you can primary hit that head i.e wide grip its the inner head of the tris more the the outter. so the result is the inner head over powering the lloutter head
that my $0.02