Define Net Book Value - MARKETING

For mutual funds and exchange-traded funds (ETFs), the Net Asset Value (NAV) is the portfolio value of the fund, per unit. For mutual funds, transactions occur at the NAV value, but for ETFs the ... What is the point of #define in C++?

I've only seen examples where it's used in place of a "magic number" but I don't see the point in just giving that value to a variable instead.