The different cache services have different advantages and disadvantages. The purpose of this website is to provide easy access to all of them at some so you don't have to painfully look it up yourself. When you're searching for an offline version of a specific site, you want to be sure that you checked all the different options the web has to offer.
The Google Web Cache is the biggest cache provider. Most of recently offline pages can be found here. The Google Web cache is the content the Google Web Search Crawler sees when scraping the web for adding pages to its index. However, lots of pages only contain the most recent version. There is no way to view older version than the last version the crawler has seen. Unfortunately, some pages can block the Google crawler and are therefore never available in the Google Web Cache.
The Archive.org cache stores multiple versions of webpages. If you're looking for an old version of a webpage, use the Archive.org cache. Unfortunately, the Archive.org crawler is also blocked from some webpages by disallowing its User Agent in the robots.txt file. A user also has the ability to trigger a custom caching action, that means that we can force the Archive.org cache to schedule a crawl of a page we want. This is useful if you want use a website for a proof.
Archive.is allows simple website mirrors which can be triggered by any user. The Archive.is crawler does not crawl on its own, unlike the Google Web Crawler of the Archive.org crawler. It has always to be triggered by a user, manually. There may be pages archive in Archive.is if a web user previously knew that a site content is likely to be taken down or changed. In this case the Archive.is page is the best choice.
Megalodon (also known as gyc.tc) is a Japanese-only cache server. It was built to enable reporters and citizens to get a "proof" of a page: the founder says on his blog that his motivation was that to hold especially newspapers and magazines accountable for their articles and posts. If a newspaper or magazine publishes a critical post and removes it, people have the ability to "save" a copy of the publicy accessible website on his site. Use this site if the target site in question is a Japanese site.