If you are familiar with the way Microsoft Windows manages its versions, you can relate point releases to Service packs that are released after a while when the product is released to the general consumption.

The basic idea for having point releases in LTS versions is to ensure that the LTS version works on newer hardware and doesn't necessary have to download a huge amount of updates when freshly installed.

The point release essentially contains the bug fixes the version has gone since it was released to the public, which includes security fixes, package updates, translation packs updates, etc.

You can read Mark Shuttleworth's blog, where he talks about the point releases of 8.04. An excerpt from his blog that is relevant:

These point releases will include support for new hardware as well as
rolling up all the updates published in that series to date. So a
fresh install of a point release will work on newer hardware and will
also not require a big download of additional updates.