"Set Relative DataSource" action makes total personalization effortless.

Some people say that true personalization is too complex and expensive to set up and maintain, but with Sitecore it's incredibly easy to setup personalization rules, configure rule-based content, etc. In general, to make some personalized content, you should perform the following actions:

1. Create a personalization rule (add conditions and actions, let's say "Set DataSource" action to point personalized content folder). 
2. Apply it to the rendering or sublayout.

While second step cannot be avoided (unless you use global rules), the first one sometimes looks redundant, as the conditions  of the most rules are very similar, especially in location-based ones. The only difference is a DataSource. The main goal of creating "SetRelativeDataSource" action is breaking such dependency and making "clicks only" personalization setup possible.

The idea is following: let the rule control only "personalized" part of the DataSource, taking the "base"  DataSource part from the control.

Let's say, you have a lot of sub-trees with personalized content for different controls: