Relative humidity is a ratio, and the denominator moves with temperature. Ninety percent at 40F is a cold damp morning. Ninety percent at 85F is a public health event. Same number, different planet.
Dew point is an absolute measure of how much water is in the air. It does not move when the temperature moves, which is exactly why forecasters use it.
Practical scale: below 55F is comfortable, 60 to 65F is noticeable, above 70F is tropical soup and your body has stopped cooling efficiently.
Dew point also sets your overnight low floor most nights, so it is doing more forecasting work than the number your app leads with.