This is pretty obvious when you think about it, but still neat.

Example:

This is a link to [RFC 1796][1796].

So is [this][rfc1796].

[1796]: https://tools.ietf.org/html/rfc1796    
[rfc1796]: https://tools.ietf.org/html/rfc1796    

Result:

This is a link to RFC 1796.

So is this.

The practical upshot is that you don't have to worry about numerical sequencing or having to rearrange things if edits are made at a later date. Definitely useful for tag wiki stuff, or any other giant blob of RFC hyperlinks that you're throwing together.