

Features and better security architecture, potentially more privacy. A more energy efficient stable fast Safari. The APFS file system, which is a better match for your SSD boot disk.

"what will I get by running High Sierra?" Security updates.Also, installs don't always work first time. You could rely on a piece of software that relies on an earlier OS and not realize it until too late. You could overlook backing up properly before wiping the drive if you value anything on it. Power interruption or even brownout could lead to partial or corrupted files during install. "What can/may/will go wrong?" Ask Murphy.Also, a reformat of the drive is necessary to use APFS. "why not just simply upgrade it as it is?" A clean install has a number of good outcomes, like stability, compatibility, and lack of cruft, and yes performance since fewer errors = more speed.
