Standard image viewers (Windows Photos, macOS Preview, IrfanView) show you the raw image. However, RPG Maker assets rely on strict mathematical grids (usually 48x48 pixels for MV/MZ, or 32x32 for older engines).
While the default image viewer on your computer will "work," it won't tell you why your sprite is glitching in-game. This tool provides that visual context. rpg maker png viewer