Torque to yield bolts.

why do modern engines use these on heads etc.just curious.

For the clamping force they offer

Torque to yield is nothing new, it goes back to the '40's and '50's and probably earlier, but I don't have much experience of pre war stuff.And just because it says to torque to 90 plus 90 doesn't mean they are torque to yield, that's just a more accurate way to achieve the desired torque value.

The bolts are made to a a very tight tolerance, therefore they can predict exactly how much force it will take to stretch the bolt 1mm (and how many degrees to move 1mm).
As you tighten bolts more and more, the friction between the bolt and the cylinder head increases. because this friction varies greatly from car to car and bolt to bolt just torquing to a high setting is fairly inaccurate as you are trying to add the torque from stretching the bolt along with the torque to rub the bolt against the head under load.

(this also varies with time, if you torque to 90 ftlbs then let it sit for 30mins you will prob get another quarter turn out of it to bring it back up to 90ftlbs as the bolt settles)

They now generally toque to somewhere like 30ftlbs which is low enough to be accurate then they will take 2 steps at 90' each. This means they will then know fairly accurately how hard it will pull the head down.

Pulling it down in 3 stages also puts good even pressure on the gasket.

3 stages also means less twisting stress on the head, but that final pull can be a bugger to start in some situations.