Users usually want to adopt a wiki in order to edit core features that normal users cannot - such as the MediaWiki pages (they are only editable by local admins and some global usergroups). The user can also add other users to usergroups (such as appointing other admins), helping the wiki grow again. Other functions include able to delete content, block users, etc.
Uh, you can do one adoption in one month yeah, but there are no rules about the maximum of adoptions further :)
That's the usual interpetation of the guideline, and my own too. But the guideline only says "You have not adopted another wikia in the last month". It's about adopting, rather than filing an adoption - so technically, a second adoption wouldn't become void until the first adoption is approved. And if the first is denied, the second still stands. But that's veering off-topic.