This is actually a two fold issue.
1) Does this site need mods.
Mods keep things "polite and nice".
Some topics get rather "emotional".
these topics are where you have people blow their lids and such.
Mods can help move topics to the correct location.
Mods can warn people they are stepping over the edge.
Mods can remove posts that are not on topic or are making personal attacks.
2) Mods are people with their own thoughts/feeling and ways.
What that means is one mod may consider a reply bad and delete it while another may let it pas and a third may write a warning in the thread.
3) Mods "should" take their cue from the site owner/administrator.
Realisticly, the site owner/admin don't really have enough time to mod all the topics and maintain the site, it's just too much work.
If the mods work closely with the admin, they are a useful or at least a necessary evil.