Why do refrigerant cans have this?

[mdredmond]

R-134a can. My pet theory is that it provides a predictable blowout location if the can is accidentally connected to the high side of an A/C system. But that’s totally made up on my part.

Could also be a “grab” location for something that rotates the can during label printing.

[TBJK]

Pretty much the blowout as well as your supposed to knock it out. Disposable cans have that. The larger tanks have actual reliefs.
Here is what one looks like on R-410a