What would be the best benefits of upgrading from 0.31 to 1.2 if i need to convince the Product Owner?

A lot has changed since then, some critical bug patches, some features that only work with recent SDKs (url redirects, sticky bucketing, prerequisite features, advanced caching & hydration). Plus we don't support versions that old. It's in their best interest to keep up to date if for no other reason than for the security & support benefits. Full changelog: https://github.com/growthbook/growthbook/blob/main/packages/sdk-js/CHANGELOG.md