It is not only cosmetic
Two things follow a cracked back. Wireless charging works through the panel, and a break in the wrong place stops it working reliably. And the seal is gone, so the phone is no longer water resistant in any meaningful sense.
The third thing is more mundane: broken back glass keeps shedding splinters, which is the reason most people finally bring it in.
Why the price swings so much by model
From the iPhone 14 onwards Apple redesigned the chassis so the back glass can be removed on its own. That made the repair dramatically cheaper on those models.
On the iPhone 8 through 13 the glass is laminated to the frame. It has to be broken out in controlled pieces, often with a laser, and the adhesive residue cleared before a new panel can be seated. The part is cheap and the labour is not, which is why the same repair can differ by a large multiple between two phones that look alike.
When leaving it is reasonable
If the crack is a single line, the glass is not lifting, wireless charging still works and you use a case, leaving it is a defensible decision. We will say so rather than sell you a repair you do not need.
If it is shedding, or the phone is going to be sold, get it done. A cracked back costs more off the resale price than the repair costs to do.