I must admit I've never used the Confluence find and replace feature. It's that little magnifying glass icon in the editor you may have seen that magically shows extra options.
I've always preferred to use the Ctrl + F option which is the find option in the browser to find search for keywords.
However, as you may already be aware, you cannot perform a replace with a browser. That is because a browser is not a text editor. (That being said, there are various plugins that allow your browser to be one and to replace as well).
That is why the replace feature is included in Confluence. This only works on the current page you are editing. If you are looking for a global find and replace though, you'll have to revert to a Confluence plugin for that!
In summary, if you're looking to "find" a word, it's quicker and easier to use ctrl + F. If you want to replace a word, hit the magnifying glass!