If a round or top-down token is the same size as a grid square, it essentially centers on the space. However, if a token is resized (such as if you have art of a tiny or small creature and want to illustrate that it isn't as big as a medium sized one so you scale it down), the grid snaps it to the top-left corner instead. This is aesthetically displeasing and with enough tokens can make it harder to accurately judge distances on the map. All tokens should by default be centered within their space.