In general, once a user has contributed content to this site, they grant SE an irrevocable license to use that content (according to the terms of the license). From the terms of service:
You agree that all Subscriber Content that You contribute to the Network is perpetually and irrevocably licensed to Stack Exchange under the Creative Commons Attribution Share Alike license. You grant Stack Exchange the perpetual and irrevocable right and license to use, copy, cache, publish, display, distribute, modify, create derivative works and store such Subscriber Content ..
So if the content is valuable, trusted members of the community are allowed to vote to restore it.
The scenario where this often occurs is when someone rage quits the site and deletes all their content, including very good answers to old questions. This is unfair; had they never posted an answer in the first place, then someone else would have probably supplied the same answer they did. But since they did post the answer, once they delete it they leave behind a question where the remaining answers are incomplete (often just expanding on points made in the now-deleted answer, so they don't even make sense anymore out of context) and this has a negative effect on the overall quality of the site. In this scenario, I will undelete the answer.
(The author of the post can always ask for it to be dissociated, so it won't be linked to any user account.)