Head bolt torque

Doing full upper gasket replacement on a 94 3.1L.

Factory service manual says to torque the head bolts to 33ft pounds then an additional 90 degrees, is this correct?

Why dont they just give a specific torque to bolt them to without the extra 90 degree turn?

If thats what the manual says believe it, why give a torque then 90deg ... make sure you replace the head bolts!, when they have you give it an extra 90deg they are torque to yeild bolts, which means the 90 deg actually streaches the head bolt a little bit, if you re-use them there is a chance you will not have the same clamping load, your head gasket will not last, and the bolts could break off. New head bolts, follow their torques, follow the order they say to tighten them in.