Number Cruncher is right.

In engineering principles the bolt is an elastic device: It's stretchy and stretchy in proportion to the force applied. So if you stretch it x% a very predictable amount of force will be maintained providing it's within it's elastic limits.
So why 90 degrees of turn? That says that the bolt has been stretched by 1/4 of a thread and that's exact measured stretch.

If you do some reading on the vagaries of torque you will be surprised. All kinds of things can interfere whilst using torque:
quality of male thread
quality of female thread
quality of lubricant
cleanliness of sliding surfaces
even temperature of lubes
All of these introduce errors which are likely impossible to predict or compensate for.

Stretching bolts by a measurable amount; 1/4 thread or any closely measurable amount is doing what you want to do; apply an elastic force of an exact and predictable amount.

__________________
Retired Marine Engineer
2005 E320 CDI
1987 300 Turbo Diesel @ 300,000 kM going
1974 240 D 300,K Miles when given away gone
1970 200 D gone
1969 190 D gone