Why do I grant access to /usr/share in apache2.conf

Solution 1:

if you are not using web applications which are packaged with your OS (or you install to that location) you can safely ignore it.

Examples of stuff which might be there though would include common PHP libraries (like ADODB etc), frameworks like Zend, programs like Cacti, Nagios, Cups etc may all be there.

There is, of-course, no harm in commenting this code out if you don't think your application uses it, and add it back if you get errors in your log of the nature that it can't read files there.

Solution 2:

/usr/share is, as the quoted passage says, "used by web applications packaged in Debian."

If you install, say, Wordpress from an apt repository, it will be installed into /usr/share.

If that's still not plain English enough, we'll need more detail about what part of it you're understanding and what part of it you're not.