I was a frequent user of SelLast in R5. It was helpful in determining the output from the last command executed (think curveboolean with keep all). The menu item explains that it is the last created obj (Select Objects -> Last Created Objects).
There are many instances where SelLast performs as SelPrev or SelParent. Steps to reproduce:
Draw planar crv
Select planar crv
It selects the planar crv which is the previous selection and the intended srfs parent. This is not limited to _MakePlanar. SelLast witll return the input from other commands as well.