Segal's Law: A man with one watch always knows what time it is. A man with two watches is never sure.
Another gauge will almost certainly give different readings than the gauge on your floor pump. If you already know which psi readings give your desired ride quality with the gauge on your pump, the only benefit of a separate gauge is in case you are filling tires with a different pump; otherwise it just confuses the issue.
If you are still dialing in the best pressures, and esp if you are using a calculator like
this one, then sure, yeah, having an accurate gauge may be useful. (Gauges that are built into pumps are usually rather inaccurate.)