Emacs 29 is the current stable release, adding major features: Eglot LSP client, Tree-Sitter language parser and package.el package install from Git repositories.
Building from source is a convenient way to try Emacs features in advance, especially useful for those maintaining or developing new tools on top of Emacs.
Emacs 29 - what to expect - a brief summary of the major features
Emacs provides a huge amount of features via packages. Community configurations like Spacmacs orchestrate packages so they work seemlessly together. It is rare, but sometimes a package will have a breaking issue.
Melpa provides (5,544) up-to-date packages automatically built from each projects Git repository. Builds are defined by a recipe for each package.
A custom recipe can be used to control which version of a package is used with the Emacs configuration, to work around issues or changes to a package.
Emacs 28.1 is a feature packed release and includes native compilation to significantly increase the speed of all Emacs software packages installed. This is a very noticeable difference, especially when Emacs is at the center of your developer workflow.
Ubuntu hasn't packaged Emacs 28.1 yet, although its usually straight forward to build Emacs yourself.