Hi Kathy, You are certainly free to update or not as you see fit. Some benefits to updating are:

Security vulnerabilities are caught and fixed on an ongoing basis.
New and improved features that you may find useful down the road.
Improvements to load speeds and performance.

You may also in time find that newer themes and add-ons are only being produced for newer versions, so you may see something that would benefit you and not be able to use it.

Concrete5 certainly wont force you to upgrade, but there are certain benefits to keeping your site updated. It is entirely up to you, as it may take some adjustments to get your 5.4.2.2 site working on 5.5.1.