My battery health is 77, the advantage and disadvantage

From: Batteries - Maximizing Performance - Apple

"A normal battery is designed to retain up to 80% of its original capacity at 500 complete charge cycles when operating under normal conditions. The one-year warranty includes service coverage for a defective battery. If it is out of warranty, Apple offers battery service for a charge." --> iPhone Battery Repair & Replacement - Apple Support

iPhone Battery Replacement:

Current Apple Prices:

• No charge for in-warranty or AppleCare+ plan.

Out of Warranty Prices:

• iPhone 14 or newer: $99 U.S. Dollars.
• iPhone X to iPhone 13: $69 U.S. Dollars.
• Older models: $49 U.S. Dollars.

Click ▷ Check AppleCare Coverage.

Click ▶︎ iPhone Battery Replacement.

If you have a battery health below 80% your battery is eligible for replacement as the health is severely decreased

It’s up to you. Until and unless the battery starts to bulge, you can choose to live with it. You will need to charge the phone more frequently or tether it to an external battery.