For example, want to get more likes through the norm of reciprocity, but don’t want to have to bother actually clicking “like” on everything? You can use a TamperMonkey script to randomly like content in a realistic, human manner.
TamperMonkey gives you access to pretty much everything you could get by writing a custom Chrome plugin, but this way you can easily manage and edit the scripts in one interface rather than needing to create and edit separate plugins.
You can also access hundreds of already written user scripts via the TamperFire option in TamperMonkey, which lets you access TamperMonkey compatible GreaseMonkey scripts from the site UserScripts.org.
Just search for the site you want a script for and see which scripts are available. As you’ll see in this screen shot for “facebook.com” scripts, most do minor cosmetic or functional one-trick pony things.
The major limitation of TamperMonkey is that it is not meant for website automation and thus you can’t (in a stateful manner) really write a script that loads X page clicks a button, which loads Y page and clicks another button, then loads Z page and fills out a form. This is critical to complex automation, where you need knowledge of what you did on X page when you get to Y page. In some cases, with dynamic web 2.0 sites which never unload the DOM tree and just update portions of it, you can automate things with TamperMonkey.
So, understand that TamperMonkey is just one tool, very useful at what it’s very good at. For other situations you need other tools, one such being iMacros.