I already run Zabbix / LibreNMS / PRTG / Checkmk. Do I need this too

Palpabl solves similar but different problems than these solutions, Palpabl serves as a documentation tool.

They solve different problems, and most teams want both. Your monitoring tool watches live state — up/down, traffic, thresholds. It doesn’t hold the intended state of your network: the documented record of what should exist and how it’s wired. Palpabl is that layer. Keep the monitoring tool you already have — Palpabl is the documentation/source-of-truth side those tools were never built to cover.