Is it necessary to fix a cracked iPhone 16+ back?

That's up to you. If you don't have the back glass replaced, the phone will have NO trade or resale value. And further damage could occur. So if it was my phone, I'd pay Apple to replace the back glass. Then I'd put pit in a good quality protective case, to prevent this from happening again.

iPhone Repair and Service - Apple Support

Do NOT let anyone other than Apple or an actual Apple Authorized Service Provider (like BestBuy in the US) touch your phone. Letting anyone else touch your phone will invalidate any warranty you may have and Apple will never touch a phone which has been tampered with if you have a service need in the future.

Repair cost:

• No Warranty: $159
• AppleCare Plan: $29

As previously mentioned, a damaged iPhone has no trade-in value whatsoever.

Therefore, I recommend service.

Service Checklist:

• iPhone service pricing.
• Check AppleCare Coverage.
• Get your iPhone ready for service.

Need more help?

• Call 1-800-APLCARE.
• Contact Support.
• iPhone Support.
• iPhone User Guide.

Click ▶︎ Check Repair Status. 🔧

Good luck! 👋🏼😉