Hi Jessica,
KISS is mainly geared for conversion tracking and ROI goals. It has been built from the roots to help you improve your customer acquisition funnels. With GA you can also do some of that, but not to such sophisticated level as KISS.
Also, KISS metrics merges with KISS insights so you can gather info not only on 'WHAT' your customers do on your site, but WHY they do it or more importantly WHY they dont do it. The results of those WHYs can inform the process and enrich the data you get via KISS metrics
I think it all depends how much disposable time you have to focus on really detailed conversion-based bunnels and ROI tracking activity in detriment of planning your campaigns, link building, technical audits, or the like... 