Lots of these changes are quite significant improvements under the hood. One thing emacs has struggled with is that it has been around for a long time so many features were added when the libraries around them were quite immature, and emacs ended up with libraries that were difficult and outdated and insufficient manpower to quickly move to newer libraries. Another issue is the desire for backwards compatibility (which can rule out libraries which eg require gtk or can’t sufficiently cope with non-UTF-8 encodings).

Adding harfbuzz gives emacs much better text shaping and let’s emacs get rid of whatever ancient library they were using before. Similarly the old dependency (and security liability) on imagemagick can be dropped.

Other improvements should just allow certain elisp code to use C functions to run faster. I think it is a testament to the versatility of emacs lisp as a language that these things were good enough while implemented in pure elisp that they could still be productively used, so no one needed to first request new C functions before they could implement e.g. lsp clients or calculators.