The more moderators there are helping out, the less work for everyone, and the faster any moderated messages will be processed and delivered to the lists. More moderators in more timezones also means there's less chance of a message sitting waiting in the queue while other timezones are sleeping. If you're able to check a moderation mailbox say twice a day, and spend something like 5-10 minutes each time browsing messages, then you can help moderate.
Note that due to the associated responsibility of being a moderator, only people that are known on the lists, or long term subscribers can be considered as moderators. This is to prevent for eg an unknown spammer signing up, spamming all lists with some junk or a virus, and then using her moderator position to approve all those spams.
If you are interested, please see the moderation guide .