i founded something funny for some users

as you notice , some sites have a catalogus , other sites dont
but those sites that dont have a catalogus , why do they enable sitemaps ??
its kinda useless to use sitemaps (with all links) if you dont enable catalogus , so google will find your links but the users cant find it on your site ??
but the other question i have
how many users have the catalogus ??
and why do other users disable the catalogus ??
Thank you
Greets From PowerChaos