Occasionally an update is important enough that it shouldn’t wait until QA is done testing its next release to be published. Less urgent updates wait for normal publishing.
Yes. You will hardly see Update used in Tumbleweed.
On the contrary, in Leap it is the normal way to offer patches/updates during the lifetime of a openSUSE version. In Leap the OSS and non-OSS repos will not change during the lifetime of a version.
So, complete different handling between Leap and Tumbleweed here.