With SA, you'll get feature updates; without it, you don't get those new features, just security patches, and eventually you'll have to pay for new Windows 10 licenses to bring your PCs up to date.
You'll also get the Windows 10 Enterprise Long Term Service Branch that you can put on mission-critical and sensitive systems where you don't want the configuration changing. With SA you get 10 years of support and new releases of LTSB that you can choose to move to every two or three years; without SA, you don't get the new versions of the long-term branch, and you only get five years of support.
That's substantially the same choice you've had before; paying for either just the current version of Windows or buying Software Assurance to give you the right to upgrade to the next version. But with the new Windows as a Service model, upgrading to get new features is far less disruptive, which may make Software Assurance a better value.
Sign up for CIO Asia eNewsletters.