Why not just set quality 70?

The same quality setting gives wildly different sizes on different pictures.

Does it keep metadata?

Re-encoding drops most of it. Clean first to know exactly what went.

Size & format — Make your images fit.

Smaller files. Same daddy energy. It finds the point where compression would start to show, and stops just before.

“Looks identical” lowers the quality step by step and compares each try with your original by SSIM, a measure of structural similarity — stopping before a difference would show.
Target-size mode encodes your actual image several times and keeps the best one that fits. A screenshot and a photo need different settings; it measures instead of guessing.
A before/after wipe at 100%, so you see the banding before your client does.
If nothing fits, you get the smallest it managed and a sentence saying so.

Everything on this page runs on your device. Nothing you drop here is uploaded.

Why not just set quality 70?: The same quality setting gives wildly different sizes on different pictures.
Does it keep metadata?: Re-encoding drops most of it. Clean first to know exactly what went.