Basically for mobile game is all about performance of the device, you have to optimize a lot for mobile games. For computers you don’t have that same problem.
If you need to optimize because you have a lot of vertices, one way is to retopologize yes, but there are much other ways to optimize, not just that one.
The sculpting step is not a necessity, you go go straight to modeling if you want, it depends on the workflow and pipeline. Normally for the more free flow initial stages you do sculpting because things change a lot, and before you commit to a model, you first sculpt because it is more flexible for changes.
