My interpretation of your question is that you would like to know why you're being advised to install ppa:ondrej/apache2 over the default Ubuntu Apache2 package. The most authoritative answer I could find to this question is from the PPA author:
Q
In the main PHP PPA it is recommended that:
If you are using apache2, you are advised to add ppa:ondrej/apache2
However, there are no arguments as to why this is recommended, what
benefits this apache build will have and what risks systems might be
exposed to by doing so...
A
apache2 has all the fancy stuff like modern crypto, HTTP/2.0 and
TLS/3.
For the rest, you need to do your own research (I provide couple of
updated modules that has been requested) and maybe submit a patch for
the description if you want to see it improved.
In another GitHub issue, the author gives a similar answer:
Q
On a related topic: I see that you recommend using the ondrej/apache2
source for Apache as well ("If you are using apache2, you are advised
to add ppa:ondrej/apache2"). Can you give me an idea of the risks of
not using ondrej/apache2 if I'm using ondrej/php?
A
The apache2 from PPA is up-to-date, has better HTTP/2 support, has
TLS1.3 support. There are no risk per se.
The reasons given should not apply to Apache2 from the Ubuntu 18.04 repos; the latest release, 2.4.29, supports TLS 1.3, and HTTP/2 should just work.
It would be helpful if the PPA maintainer updated the FAQ page to this effect, and perhaps elaborated a bit on when this advisory is actually applicable.
