They don't call duct tape "100 miles an hour tape" for nothing, this versatile adhesive comes in several colors and can be used out in the field or at home for repairs.
For Ranger School, this is a mandatory item, and you are required to bring one. I would strongly recommend you bring two rolls.
For Sapper Leader Course, this is a mandatory item, and you are required to bring one. I would strongly recommend you bring two rolls.
For SFAS, this is a optional item and you are authorized to bring two rolls. I strongly recommend you bring some.
You will use this for everything from making a Ranger/Sapper purse, to fixing your pants when they rip.
A good trick is to wrap some 100 MPH Tape around a lighter (see pictures). This way you can quickly grab it if you need some without having to carry the roll in your pocket or having to stop and dig it out of your ruck.