If you’re in the midst of some kind of crisis, whether physical or mental, it’s somewhere safe you can go that’s available 24/7.

From there they can keep you safe until you’re stabilized (physically or mentally) and then refer you to specialists or hospitalize you.

The ERs I'm familiar with would look at you, see that you're not bleeding out or having a heart attack or stroke, and then you'd sit in the waiting room for six hours to finally be seen by an intern who would send you out with a psych referral (probably booked out months in advance).