Beta weighting restates a mixed book in one currency: "if the index moves 1%, I make or lose this much". Each position's delta exposure is scaled by its beta relative to the chosen index and by relative price.
Example: a portfolio shows +$180 of profit per 1-point move in SPY at SPY 500. A 1% index move (5 points) is therefore about +$900. On a $60,000 account, that is a 1.5% swing per 1% index move, so effective market exposure is roughly 1.5x - useful information that no position list makes obvious.
Options traders rely on it because raw deltas across different underlyings cannot be added. It is an approximation: beta is unstable, and it breaks down precisely in crashes, when correlations spike and option deltas move fast. Treat it as a dashboard gauge, not a guarantee.
Related: portfolio-beta, delta, net-exposure, effective-leverage