Whether or not you require 3rd party apps depends entirely on your needs and goals. The app you're referring to automatically creates your product feed for Google Shopping campaigns and syncs products from your Shopify store, but you need to consider what other functionalities you might need to actually manage the product feed.
For example, do you want to use rules to automatically exclude out-of-stock products from your feed? Do you want to handle multiple feeds - Google Shopping, Facebook, Amazon - from the same app? Feed management apps help optimize your product data and drive your conversions, whereas the Google Shopping app is fairly limited in that it only generates your feed for you. Some apps also offer feed audits and recommendations from campaign experts to enhance your feed, which you may want to consider.
Hope that helps
Thanks guys - great responses.
@Merryn - "do you want to use rules to automatically exclude out-of-stock products from your feed? " - I believe G SHopping feed does this automatically if a product is no longer in stock. That's what is showing me in my console at least...
I still see a use for G Shopping feed handlers, however, given our desire to only worry about G Shopping channel for now + lack of need for too much customisation, I think the basic app is fine.
Remember with feed apps you get a lot more than just your product feed. For example, you get different filtering options, label support, channel support and integrations. ShoppingFeeder is a feed app that can create your product feed not just for Google but also for Google local ads, Facebook, Bing, Pinterest, Kelkoo, the list goes on. ShoppingFeeder integrates with Google Merchant Center, but also with Google Ads so you can run your smart shopping campaigns through the ap, which is pretty cool. You can find them on the Shopify app store or on their website
Just so you know how broad your options are, hope that helps