Reasons:
1. Archives - those tutorials and guides stay when the original pages go 404
2. API - I use the api to automatically post my bookmarks to my blog
3. Full-text search: this is very very useful when needed
4. Social Discovery: Search that niche website / app on Pinboard. It shows lots of other people who found that same thing as interesting. We can then follow them and subscribe to their favourites as RSS feed.
