Beta is needed to estimate the amount of additional reward you will receive for purchasing a risky asset instead of a risk-free asset.

Risky Asset

An asset whose returns can vary considerably and are not predictable, presenting a higher chance of loss.

Risk-free Asset

An investment that is assumed to have no risk of financial loss, typically represented by government bonds.

Beta

A measure of a stock's volatility in relation to the stock market as a whole, indicating the risk associated with investing in a particular stock.