A 60 per cent gain means nothing until you know whether it took two years or twenty. Compound annual growth rate converts any total return into the constant yearly rate that produced it, which is the only way to compare investments held over different periods.
The arithmetic average overstates what you actually earned. Up 50% then down 50% averages to zero but leaves you down 25%. CAGR is the geometric mean and describes the rate that actually turned your start value into your end value.
Very, between roughly 4% and 12%. Divide 72 by the rate for the years to double: 8% gives 9 years against an exact 9.006. It drifts at the extremes but stays close enough for mental arithmetic.