Also, that they often simply don’t sell what you want with enough memory, or pair memory upgrades with other upgrades you don’t need (e.g. more powerful CPU or GPU beyond your needs), or occasionally that you actively don’t want (e.g. iGPU → dGPU may be that). With socketed RAM you can buy the model you want that just lacks RAM, and upgrade that.

My current laptop (ASUS GA503QM) had 8GB soldered and 8GB socketed. I didn’t want to go for the 16+16 model because it was way more expensive due to shifting from a decent GPU to a top-of-the-line GPU, and a more-expensive-but-barely-faster CPU. (I would have preferred it with no dedicated GPU—it would have been a couple of hundred USD cheaper, a little lighter, probably more reliable, less fiddly, and have supported two external displays under Linux (which I’ve never managed, even with nvidia drivers); but at the time no one was selling a high-DPI laptop with a Ryzen 5800H or similar without dedicated graphics.) So after some time I got a 32GB stick and now I have 40GB of RAM. And I gave my sister the 8GB stick to replace a 4GB stick in her laptop, and that improved it significantly for her.