Hey there!
If you can’t think of a utility for SGTM, then you don’t need SGTM 
It’s definitely not a one-size-fits-all solution, and it does benefit from a deeper integration into the Google stack.
Using Snowplow as the entry point and then utilizing other vendors’ tags to fan the SP request via SGTM is a great way to make especially client-side measurement leaner, but if you’ve already got a solution for that via webhooks you’ll at most miss out on the easy configurability of a GTM container, which might not be worth the investment of time and money.