The main reason you should consider developing or using a bookmarklet is for productivity.
How to install a bookmarklet?
There are many ways to install a bookmarklet. I will show one of them which is probably the simplest one.
Search for bookmarklets you want to install on the internet.
Drag the bookmarklet from the page to your Bookmarks Toolbar. It should show on the toolbar now.
If the Bookmarks Toolbar is not visible then you first have to show the Bookmarks Toolbar by right-clicking on an empty section of the Tab Strip and checking the Bookmarks Toolbar in the pop-up menu. Now repeat the above step.
How to use the installed bookmarklet?
Go on a website where you want to use your bookmarklet.
Click the bookmarklet on your Bookmarks Toolbar. You will see the magic happening.
You can find many more pre-built bookmarklets on the internet.
I hope this helps someone out there.
Don't forget to mention what you've made in the comments below!