Any single currency pair only tells you about the relationship between two currencies — EUR/USD rising could mean the euro is strengthening, the dollar is weakening, or some mix of both. Currency strength solves that by measuring each of the 8 major currencies against a basket of the others, isolating which one is actually doing the moving.
This matters most when picking between pairs that look similar on the surface: if you're deciding between two pairs that both involve a currency you think will strengthen, checking which of the two counter-currencies is structurally weaker can be the difference that actually matters for the trade idea.