[2019] Was there a reason why we needed new shell types?

What we can call SAP really depends on a nation.
Most of the time SAP shell is basically an AP shell, with some modifications - most if the time it has bigger bursting charge. In some cases (German, if I remember correctly) SAP - is just an HE with bottom fuse. Also, I've read different shell manuals, and even one nation can call the exact same shell AP in once case or SAP in another.

Why not? It's a shell type. It existed and was pretty wide-spread.

It has higher damage potential, better ricochet angles, can't overpen, but reduced penetration - basically hitting citadels should be limited to very specific cases.