Putting Something on the Web Makes It Accessible and Public

"Putting something on the web", in this context, means putting it on a website or on a web application, though there are other ways to interpret this.

The accessible part is the main reason to put it on the web. By putting something on the web you make it available across different devices and computers.

The public part is what you get by default. It's accessible, and if you don't take any extra steps, anyone can read it. If you don't want this, you can lock it down and you can still get the accessibility without the public part. This would make it more of a web application.

Sometimes you want something to be accessible and you don't care if it's public, in which case the public aspect is just a side effect. My bookmarks would fall into that category.

Other ways to be accessible

You can make data accessible without putting it on a web page. You can, for example, put it in a Dropbox instance, and it would be accessible without being public. This can be an alternative to making it a web application.