Its advantage is timing and comparability: the flash is the earliest hard survey read on the current month, and the same methodology runs across the US, the euro area, the UK and Japan, which makes cross-country comparison meaningful.
It often diverges from ism-manufacturing-pmi because the sample, the weighting and the seasonal adjustment differ, and S&P Global's panel skews toward smaller firms. Neither is right; they measure slightly different populations.
Example: euro area flash composite rises from 49.6 to 51.2 while the US flash slips from 54.1 to 52.3. The relative move is the tradable signal, and it usually shows up in the currency pair before it shows up in rates.
Related: pmi, ism-manufacturing-pmi, ism-services-pmi, diffusion-index, nowcasting