The main use of the solver is for something that definitely cannot be done without programming vba macro. You use it to find the optimal value in a cell based on the result in a different cell. You have a cell that excels will solve, a cell with the objective and constraints.

For example, let's say you have a spreadsheet to calculate the capacity of a steel column based on its height and cross-section/material. If you wanted to run the calculation backward and find the height that will give you a very specific capacity, you could do it manually by playing with the height until you find it or you can let Excel do it for automatically with the solver.