Redirects are used to forward the user from one page on the wiki to another page.
We use redirects to connect logical search phrases to helpful places. They don't cost us anything, so they're highly-encouraged. There are a few exceptions, but unless one redirects to a place to where someone wouldn't want to go, there's no downside to having them. In such a case where a phrase could have multiple meanings, a disambiguation page is used.
For example, let's take a look at the search phrase "conkers bad fur day". If it did not exist, users would be taken to this results page. MediaWiki's built-in search is not great, and redirects allow us to make up for that by applying a human touch to point users in the right direction. If there's even a small chance that a redirect will help someone, it's not useless.
Due to MediaWiki's search irregularities, a proper-capitalization redirect and all-lowercase redirect may exist for each topic (for example, Northern Kremisphere and northern kremisphere). Deleting redirects provides no real benefit, so they should only be tagged for deletion in the rare cases where (1) they are completely nonsensical or (2) they are too general. There is no need to delete alternate name, conjectural, or spelling-mistake redirects unless they are specifically too silly or too general. When articles and galleries are moved, redirects play an important role so that links from search engines and external websites continue to work.
(1) We don't need the redirect Game with banjo and airplanes because it's too silly.
(2) We don't want the redirect First conker game because we're not positive what page the user is looking for (you might say Conker's Pocket Tales, but maybe they're actually trying to find Twelve Tales: Conker 64), so the search page is the best place for them to be.
Other types of redirects
Template, Category, and File redirects are discouraged. In the rare case that one of these types of pages is moved, it is necessary to update all links to the new name and delete the redirect page.
Smart apostrophe/quote redirects
Due to certain devices using smart quotes and apostrophes by default (“...” and ‘...’), searching a title that uses regular quotes and apostrophes won't direct those users to that page unless they use the correct type, so creation of smart apostrophe/quote redirects (for example, Conker’s Bad Fur Day for Conker's Bad Fur Day) is encouraged.
Linking to redirects
Linking to redirect pages is discouraged, so always try to link to the actual page name and use the piped link formatting to correct the name if necessary.