An internal hard disk drive (HDD) works differently from a solid state drive (SSD), but may be the better option in some circumstances.
The primary benefit of SSDs is their speed, but they are much more expensive and have less storage capacity.
For those that need a lot of storage, or who want to keep their data for a long time, an HDD may be the better choice as they have far greater storage capacity and there is no limitation on the number of write cycles.
SSDs are less robust than HDDs as they have no moving parts, and have limited erase cycles.
An HDD may be easier to fully erase and is compatible with older computers.
The best solution may be to have both an SSD and an HDD in the same machine so that speed and storage needs can both be met.