Why do the PhpED updates always require a restart of the whole computer?

While the requirement is certainly annoying (especially so since I don't remember there being a warning before running the install that a restart will be necessary), I don't intend my question to be interpreted negatively, but rather, I'm curious. PhpED isn't unique in this respect; restart after install/update seems to a very common requirement of Windows software. Coming from a Mac background, where it's extremely rare that a restart is required after an install (except of core OS components, and even then, you can usually get around it if you really want to), the frequency of the requirement has really surprised me. Is there some common action that apps like PhpED are doing to the OS that necessitates this, or is it just the default behavior of the installer, or...?

this is a bug in Inno Setup installer. It was reported quite a long ago but it's still not fixed. You may ignore this request to restart.

See some recently posted similar issues below:
http://news.jrsoftware.org/news/innosetup/msg63798.html
http://news.jrsoftware.org/news/innosetup/msg63807.html

for the tech details see the explanation below:
-phped installer has many built-in installers for php, php5, mozilla etc.
-latter one (the mozilla) has to run uninstaller first, or old components remained from the previous install may cause mozilla failing to work as it blindly auto-registrers all the components it finds in its directory.
-when uninstaller finished, it has to remove itself so it registers an entry in the registry for later deletion.
-at the end of its work the outermost (phped's) installer sees that entry in the registry and asks for reboot.