Because you can't go back! As with the other two tactical Braveland games, saving is automatic and you only get that single save state per play through. You cannot undo your actions (short of immediate game closure before it saves) or load a save state from however many battles ago. And since both battles and gold are limited, particularly in Hard mode, it is better to plan ahead and not waste your money purchasing equipment you will not use. Or, as is the case with some of the equipment, make a story choice that bypasses a piece of equipment you want.

This is a simple chart listing all of the equipment in the game, stats, location, and costs if applicable. Use it to help plan ahead. Or don't and just play the game. I'll lose no sleep over it.