Sandboxes are provisioned platform (NetSuite and Salesforce) accounts where you can test configurations, train end-users and most importantly, test upgrades before releasing them in to a production environment.
All upgrades of SuitePOS are performed by the retailer. As such, the responsibility rests with the retailer to perform adequate testing. Because of this, sandbox testing is advisable before releasing to production in order to make sure the unique configurations for SuitePOS you have work in production.
Are you a Sandbox candidate?
This will entirely depend on your retail environment. If you have a larger number of terminals (10+), more complex processes, higher transactional volumes, have other integrations (and customizations) or POS is deemed mission critical - than yes, you really need to consider setting up a sandbox.
NetSuite Sandboxes are separate NetSuite instances that reflect a snapshot of your production instance at any moment in time. This is very different from the "Release Preview" since Sandboxes are available all the time and Release Previews are only available just before a NetSuite upgrade. Once a Sandbox is created, you can refresh your data from production at any time. NetSuite Sandboxes are purchased directly from your NetSuite Account Representative.
Salesforce Sandboxes are separate Salesforce organizations that reflect a snapshot of your production instance at any moment in time. Once a Sandbox is created, you can refresh your data from production at any time. Salesforce Sandboxes are created from your Production org.
SuitePOS Sandboxes are purchased for your SuiteRetail Account Representative at a fraction of the cost of the production service.