What's with manufacturers selling laptops with BOTH SSD and HDD

It kind of makes sense to me. I would assume it's because you put your OS and applications on the SSD for faster load and then to preserve the integrity of the SSDs write limits, store working files on the HDD.

The laptop I am typing on now came with both an SSD (NVMe) and an HDD.

At the time it seemed to make sense economically (where could I afford to put Steam games, Lightroom images, etc.)

I had the HDD fail and replaced it with a SATA SSD. I think now the pricing of SSD is low enough that a laptop HDD doesn't make sense, particularly when you figure the HDD is vulnerable to motion and vibration.

It's a cheaper way to combine "good enough" capacity and "good enough" performance.

It is possible to configure the SSD as a "cache" of the HDD, so the combination simulates a very fast and very big disk.