I'll be honest, almost every store that asked this question needed this app.
The structured data built-in to most themes is broken and is missing critical data. There's also the risk of changes to your theme breaking your structured data and losing your Rich Snippets.
So unless you've already reviewed your structured data and compared it to Google's guidelines, you'll probably benefit from the app.
If you still want to check, I'm happy to run my Structured Data Audit on your store. Just use the contact link to the right to contact support and ask for the Structured Data Audit. It's 100% free.