You can certainly tell if the tire pressure is grossly low, like low enough to risk pinch flats. I agree you can't (I can't, most of us can't) tell the difference between 80 psi and 100 psi by feeling the tire, but you can (tell the difference) when you ride on the bike, so then you can pump it up a bit more. And if you can't tell the difference when riding, it probably means your typical riding is such that it doesn't really matter.
A floor pump with a gauge is nice to have. But it is not essential. And I'll bet many of those pump gauges are inaccurate anyway.