The benefits of using a regular expression instead of a fixed word is that the possibilities are vast. A regular expression is a compact way of describing sets of strings which conform to a pattern.
For example, if you have users on your forum repeatedly ending forum posts with a mantra like Sent from my XPhone using my app you can easily define a single regular expression that matches that kind of comment.
You can also define patterns to replace that matches all URLs, all email addresses or whatever you can think of.
Settings are made within Options->Censoring (At the end)"
Note that this is an advanced add-on that requires some knowledge of using and testing regular expressions. Improperly used it may replace text in posts that you didn't intend to replace. If You are not sure about exactly what the regular expressions that you define are doing, you are advised to ask someone who knows.
This is a very new add-on and it is in Beta stage. I've only been trying it out on my own web site but with good results. Try it out but check that it's working so that it's replacing text that it shouldn't. In the beginning I recommend that you enter a replacement string in the options, just so that you can see that something has been replaced. There are some examples provided that you may use but you should check by yourself that they are working as expected.
The replacement is occurring at the moment when someone makes a post. It doesn't alter old posts. If You need to clean up your old posts I recommend that you have a look at the nice add-on Post Content Find /Replace.