1-you don't... standalone (perpetual license... install with a serial number) is not part of the Cloud
2-you can't... Cloud programs require monthly payments
3-only if you want NEW features that are not in the program(s) you have
Some general information
Cloud Plans https://creative.adobe.com/plans
-and subscription terms http://www.adobe.com/misc/subscription_terms.html
-what is in the entire Cloud http://www.adobe.com/creativecloud/catalog/desktop.html
http://helpx.adobe.com/creative-cloud/help/install-apps.html to install or uninstall
Cloud Getting Started https://helpx.adobe.com/creative-cloud.html
Many thanks. Hugely appreciate the succinct answers. Just one point of clarification. When you say "new" features, do you mean Lightroom 6 release would only be available through the cloud? So, I could still use my 5.5, but any of the 'new features' that might be in LR 6 I would not be able to use.
Thanks for the links! I'm diving into now.
All updates start here and select product to see if what you have has any updates, read to see if you need to install updates in number order, or if the updates are cumulative for the individual product http://www.adobe.com/downloads/updates/
My understanding is that NEW features are Cloud only, so anything you find at the updates link will be bug fixes or MINOR enhancements, not anything really new
If you must have NEW, you need to subscribe to the Cloud