When you update a world that is active with users all of the current instances stay active and any new users who join those instances are presented with terrible performance. Sometimes it is so bad you need to just kill the VRC client.
This is a terrible impression to leave on new users or ones new to VRC. It also makes updating a world in time for an event a chore.
This is not to mention just simple issues with the world not being in sync visually/structurally/functionally even if it did not run terribly.
Obviously it irons out over time. But as long as an instance is open and active that was started with an old version there is a chance all new users will end up in it and depending on the severity of differences in the versions they may end up in a world that actually basically makes their client unusable.