SIPC exists for one narrow scenario: your broker fails and customer property is missing. A trustee is appointed, customer accounts are reconstructed, and SIPC funds make up the shortfall within statutory limits. In most liquidations the bulk of customer assets are simply transferred to another broker and SIPC money is never needed.
What SIPC does not do is at least as important. It does not refund losses from bad trades, worthless stocks, fraud in an investment you chose, unauthorised trading claims, or futures and forex positions outside a securities account. It is not the fdic and it is not an insurance policy against being wrong.
Every broker-dealer registered with the sec must be a SIPC member unless it deals only in narrow excluded products. Limits are described in sipc-coverage-limits.
Related: sipc-coverage-limits, fdic, excess-sipc-insurance