That’s not my understanding. Being always in a backup mode even when power is available, ensures there’s no switching delay if power were in fact lost. I don’t know that this is true with consumer grade, and suspect it’s not.
With consumer-grade stuff there's typically (but not always) a bypass of the transformer straight to utility, which then gets switched to transformer upon loss of utility. With Enterprise, unless you are running in "Green" mode, the power is always running through the transformer, with corrections (trim/boost) made as necessary. This is not "backup" mode, it's AVR (Automatic Voltage Regulation), and typical for any of the APC Smart-UPS (and higher) series UPS's.
The "delay" with consumer gear is the UPS switching from bypass to transformer. With a Smart-UPS, assuming you aren't using "Eco" mode, the power is already going through the transformer, the switching happens on the feed side, buffered by large capacitors, so there's basically nothing experienced by the equipment.
You can fine-tune these settings on the Smart-UPS series UPS's, like where you want AVR to step-in, with upper and lower bounds, and you can also change the sensitivity of what triggers a transition to battery on the feed side.
What purpose would bypass serve, on an enterprise UPS? It would already be the case in your scenario.
Anything for the green Gods, right? LOL! The "Green" or "Eco" mode bypassing the transformer reduces electricity consumption. Most equipment is not super sensitive to input voltage, like a computer for example, since their power supply is already heavily buffered with large capacitors, so they have no problem riding through the switch from utility to transformer. If you have sensitive equipment that cannot tolerate this, you simply disable that function. By default, IIRC, it's off.