By doing so, anyone who knows the passcode will also be able to make changes to the restrictions and potentially disable them without your knowledge.
The Restrictions passcode should be different than the one used to unlock the i Pad as it provides a second level of security.
The same restrictions also apply to the i Tunes Store, so movies of a restricted rating cannot be purchased (or even previewed).
The same type of restrictions can be applied to music and TV shows, allowing owners to block access to any music or podcasts containing explicit content or TV shows that display a caution.
This is useful if you'd like to set some temporary restrictions without worrying about losing any data within apps that would fall outside of any restrictions you might set.