JPEG and the usual mode of WebP are lossy: at each save, fine detail is simplified according to a quality setting. At a high setting the change cannot be seen; at a low one, blocks, halos around edges and banding in smooth areas appear. PNG is lossless, which suits flat colour and transparency and makes photographs heavy.
Compression is the tool for meeting a marketplace's limit on file weight without shrinking the pixel dimensions. Lowering the quality setting a little cuts the weight sharply, long before the picture looks worse. Heavy files also upload slowly, and on a weak connection may not finish at all.
Each lossy save discards a little more, so an image that is opened, edited and saved again several times degrades step by step. Editing from the original and compressing once, at export, keeps the loss to a single pass. Fabric texture, fine print and hairlines are the places where over-compression shows first.