Workloads that require low latency, fast random access, don't involve too much (re)writes, mobile environment, money is not an object: SSDs.

Workloads that involve lots of writes and especially rewrites, transient and ephemeral data, don't require low latency or fast random access, money is a factor: HDDs.

The key thing is that HDDs have a theoretically infinite amount of writes compared to SSDs which have a finite amount. Certain workloads are consequently going to be more suited for HDDs.

I’m not the person you answered but SSD: boot disk, cache, video files you’re working off of. HDD: data you don’t mind waiting for such as video and audio storage, backups, or just cases where you have a lot of data and the SSD isn’t yet cost effective enough