This add-on includes a variety of perks and benefits. Here's exactly what's on the Metro by T-Mobile website:

Google One Membership

• Plenty of space: With 200GB of cloud storage you have plenty of space to store all your high-resolution photos, videos, and other important files. You can even share your storage with up to 5 additional people.
• Automatic Backup: Keep the photos, videos, contacts, and more on your phone automatically backed up with Google One. And if you break, lose, or want to upgrade your device, you can seamlessly transfer everything from your old phone to your new one.
• Additional Benefits: Also included in your membership are access to Google experts and exclusive member benefits, like a Google Play Credit and 3% credit back on Google products.
• Learn more here or download the app now at Google Play. Not available on iPhone.

Scam Shield Premium

• Your Ultimate Call Control Center: Scam Shield™ app is a free app that lets you manage your anti-scam protections, so you can identify, block, and report likely scam calls.
• Know Who’s Calling: Caller ID reduces unidentified calls by displaying a caller’s information, even if they’re not in your contact list.
• Take Total Control: Optimize and organize your mobile life with control over your calls and voicemails. Get information with reverse phone number lookup and get voicemails texted to you automatically.
• Learn more here or download the app now at Google Play or Apple App Store.

I personally think it's probably best to just get the regular Google One add-on for $1 per month and to save yourself the $24 per year.