I used to be a pentester, so it was pretty applicable...

Pentesting really requires "full stack" systems knowledge, from networking, to OS, to API analysis, malware analysis, binary reverse engineering. OSCP is an applied cert, so it really forces you to be able to leverage knowledge in a practical way.

However my favorite benefit of offsec is really being able to see things from an attacker's perspective. This has been super valuable in many situations and I've found I'm many times the only person in the room with an awareness of an adversarial mindset.