Support for the "auto" keyword in contain-intrinsic-size lets web sites use the last remembered size of an element (if any), which providers for a better user experience as elements have size containment turned on and off, e.g. through content-visibility: auto.
Without this feature, web developers have to guess at the rendered size of the element; when used with content-visibility: auto, this may lead to elements "jumping around".
Explainers: https://gist.github.com/cbiesinger/f2378dbcd215495c3a1daf9696a8e91f