Google “vented downspouts”: https://frenchdrainman.com/how-and-when-to-vent-your-underground-downspout-system/

https://structuretech1.com/dont-connect-downspouts-directly-to-yard-drains/

I live in NJ and we have gotten more and more torrential rains - so several of our downspouts that carry a large volume of water and our sump pump ejectors have the vents - i guess keep an eye on your gutters if they overflow the water volume is too much to get through the down spout and you should get it looked at

@Anna (6B/7A in MD) -- I appreciate the reply.

https://frenchdrainman.com/how-and-when-to-vent-your-underground-downspout-system/ seems to suggest that it's an industry "best practice" to vent the downspouts.

Very few homes in our neighborhood have these vented downspouts. I'd say it's less than 5%.
And, these are multi-million dollar homes built by high-end general contractors.

Is it the case that all these builders just "got it wrong" during the building process? Or, did they cut corners to save, maybe $200 on materials and labor (doubtful)? Or, is using vented downspouts case-specific (i.e. some applications need them, others don't)?

I should also mention that we have an interior basement French drain and a sump pump. Not sure if that matters here...

I would think it would allow air to escape to allow the water to flow properly.

https://www.bing.com/search?q=Vent+on+Gutter+Downspouts&qs=n&form=QBLH&sp=-1&pq=vent+on+gutter+downspouts&sc=0-25&sk=&cvid=82C8986A36B445AB986A03E3D93377D3

Also the vents are removable to clean out the pipes if they become clogged.