SSI woes

Hi all,

In order for apache to set up viewing of ssi on each site, how come
you just can't vi /etc/httpd/conf/httpd.conf and add includes by
either using

DiretoryIndex index.html index.html.var index.shtml

or

AddOutputFilter INCLUDES .shtml

I was told this won't work for all sites, that you have to
individually configure some file (not sure which one) for each site.
Does anyone know why?

Thanks!
Cameron