Most ERs would be well served by opening all-hour urgent care clinics directly adjacent, and triaging the roughly 70-80% of people with non-emergent and/or minor issues into that wing rather than to the ER.
Where I live, it's practical for me to go to an ER in at least 3 different hospital systems. If one of them offered 24-hour "urgent care" I would definitely go there vs the one closest to my house.
No. The ER and its expensive specialists (e.g. trauma/accident specialists) can be reserved doing their actual specialized jobs (taking care of patients injured in car crashes, for example) without being clogged by some dude with a toothache. The ER only handles the real hardcore cases that can escalate to death in a matter of seconds, and the urgent-care facility only handles the "everyday" harmless stuff, which needs less and especially less-certified/experienced and thus cheaper staff.