Understanding fixed-income risk and return

[abbas_sukhsorwala]

So in this chapter it says that, an investor who holds a fixed rate bond to maturity will earn an annualized rate of return equal to the YTM of the bond when purchased. And this also has coupon payments reinvested. So my question is that, WHY do the coupon payments need to be reinvested for the bond return at maturity be equal to the YTM?