It is called the "torque-turn" method.
One reason is to eliminate the change in torque values due to different amounts of friction. At the lower initial torque value the effects of friction are less apparent than they will be at the final required setting. If threads are clean or dirty, dry or oily, or any other variables, you will still hit close to the correct fastening strength.
The initial torque gets the bolt "tight" The turn gives a percent of stretch to the bolt to keep things together.
When doing these, use a marker to mark where the head should end up. That way if you have to re set your wrench, you don't loose track of where it needs to get to.
Hopefully this makes sense.
Some bolts on my heavy equipment have values like 300 ft/lbs + 180 degrees. Eat your Wheaties for those.