The usual culprits are virtual machines, large images, video editing. A little less common: big data, large matrices, scientific computing in general.

I never hit 16 GB on my laptop (I even disabled swap) but I got close once with three browsers open (I segregate some web apps into different browsers), a few VMs, some other random application running. It made me think if it was time to buy the extra 16 GB I can fit into my laptop. I just checked, it's about $100.