Apart from its main purpose, what is bitcoin network used for? In the Ethereum Homestead docs there is a paragraph


  But as bitcoin began attracting greater attention from developers and technologists, novel projects began to use the bitcoin network for purposes other than transfers of value tokens. Many of these took the form of “alt coins” - separate blockchains with cryptocurrencies of their own which improved on the original bitcoin protocol to add new features or capabilities.


Questions are: 
1. What are the best examples of such 'other purposes' and projects, especially the ones that are not altcoins?
2. What are the most prominent altcoins that haven't lost their significance?
What are the best examples of such 'other purposes' and projects, especially the ones that are not altcoins? 


Things that do proof of existence like OpenTimestamps are not for transfering money but rather committing something (in this case, a hash) to the blockchain so that it is essentially preserved forever.