How many times did you found yourself in this situation?
For example, you could have a car model list, where you want to show the avaible parts and resellers that sell these pieces.
It's not very hard to do, but you have to create an extension every time you want to do someting like that.
Linked Select helps you, offering a redy-to-use module, you only have to insert the data, after that the component will handle every request and will show correct values.
You can directly add the data or, if you are expert users, you can query them directly from database. In this way, you can show Joomla! related information, for example articles in every category written by a specific user.
In a bunch of words, it's a simple tool that can help you in a lot of situations.