Meta is the core of the site, plain and simple.
Meta is where site policy is shaped, important matters are brouhtbrought up, actions are discussed and community consensus is written. Moderators, by necessity, need to be aware and involved in all of these things.
Moderators execute the will of the community and follow the procedure the community decides upon. Moderators give valuable, experienced input on discussions where policy is shaped. YhisThis makes meta incredibly important for site moderation - by moderators and the community alike.
My personal opinion is even that meta should take a more important role in moderation-interested users than it has right now.