HP -28S Manual page 306

When you use the HP-28S as a fidirect formula entry calculator,fi each
result that you compute is retained on the stack, which takes on the
role of a fihistory stack. fi This allows you to save old results indefi-
nitely for reuse later. It also allows you to break up large calculations
into smaller ones, keeping each partial result on the stack and then
combining the results when they are all available. (When carried to
the extreme, this is the essence of RPN arithmetic). The stack provides
a much easier-to-use and more powerful history stack than the single
firesultfi function available on algebraic or BASIC calculators.
A key feature of the HP-28S is that you really don't need to concern
yourself over whether RPN logic is better or worse than algebraic
logic. You can choose the logic that is best suited for the problem at
hand, and intermix algebraic expressions with RPN manipulations.
C: Notes for Algebraic Calculator Users 305