Aspect ratio requirements show up constantly, a square Instagram post, a widescreen video thumbnail, a portrait-oriented print, and the wrong approach to hitting that ratio is one of the most common ways a perfectly good photo ends up looking subtly (or obviously) wrong.
Cropping removes area, resizing changes proportions
A crop keeps every pixel that remains in the frame at its original width-to-height proportion; it simply cuts away parts of the image outside a chosen rectangle. Stretching, by contrast, independently changes the horizontal and vertical dimensions of the whole image to force it into a new ratio, which distorts everything inside the frame, a round object becomes oval, and a person's face becomes visibly wider or narrower than in reality. Whenever a photo needs to fit a new aspect ratio, cropping is almost always the correct approach; resizing without cropping should only ever scale both dimensions by the same factor.
Choosing what to crop out
The challenge with cropping to a tighter or differently shaped ratio is deciding what to sacrifice, since you're necessarily removing part of the original frame. Crop Image lets you set an exact target ratio (1:1 for square, 16:9 for widescreen, 4:5 for a portrait social post) and then reposition the crop rectangle over the subject, rather than defaulting to a centered crop that might cut off a subject standing off-center or lose important detail at the edges. It's worth keeping a margin around the main subject in the original photo specifically so there's room to crop into different ratios later without cutting anything critical.
When you actually do want to change the canvas without cropping
If a photo needs to fit a different ratio but you don't want to lose any part of the original image, the alternative to cropping is extending the canvas and filling the new space, for example adding solid color or blurred padding bars around a photo rather than cutting into it. Image Border Generator can add padding around an image, and Add Shadow to Image or Device Mockup Generator are useful when the goal is presenting a photo inside a frame rather than reshaping the photo itself.