The signal backtest applies your list of entries to historical data and shows how it would have performed. Instead of relying on intuition, you see accuracy, Gale usage, cycles and performance by asset before risking real capital.
The backtest is the step that separates an idea from a tested strategy. Use it whenever you receive, build or adjust a signal list, before putting money on it.
In practice, the backtest changes the question from whether a list looks good to whether it actually held up. Run it on a long period, with the Gale setting you plan to use, and read the per-asset results. The goal is not a perfect curve but an honest picture: where the list works, where it fails and how much it costs when it fails, so you can decide with numbers.