Confused about expressions in Prime RPN stack

[mchris]

In RPN mode, everything you do is related to the stack, so I can not figure out what exactly you want to do with an algebraic expression in command line. Let's assume that you want to find a root of an expression with FNROOT, first you enter 'your_expression' to the stack, then you enter the variable ex: 'X', and then enter FNROOT(2), for there are 2 variables on the stack for the function FNROOT to pick. I can't imagine in what situation 'your_expression' would be of any use in command line in RPN mode.