Well, I have worked as an Administrator of over 70,000 registered users for the past few years and have been a moderator on numerous sites. I know from experience that I do not always use the "infraction system" (vB) and would prefer to manually ban users. On some sites, some people do not use the warning/infraction system and instead just warn via Private Messages (Conversations) and then usernote it, so they manually ban users via Moderator Panel/Admin Panel.
Alternatively, if the user or parents of the user request their account deleted (but you don't allow accounts to be deleted, so they want it banned), it's much more convenient (in my opinion and from personal experience) to just have the ban feature for Moderators.
I'm personally in favor of allowing Moderators to ban as on all of the sites I've worked for, and the one I'm still am as an Administrator, moderators have always been able to ban as default. Just like others have said, what if the admins are not on, and somebody is attacking your site with many accounts (I've had this happen many times). The moderator should, in my opinion, be allowed to ban the accounts to put an end to the attackers, at least temporarily until an admin can get on to do IP banning. Using the warning system, you have to worry about how many points equals the ban you want and in an emergency, it's sometimes just as quick to have the Mod Panel open and ban them. I've always kept the Admin/Mod panel open in another tab for quick access.
For those site owners that do not wish for Moderators to be able to ban, it could be a simple permission in the Moderators group to allow/not allow them to ban users. If they can, it appears in the Mod Panel.