You tested low 300s multiple times that's clinically low, so 120mg isn't shutting yourself down for natty levels it's just TRT for a real deficiency. You're fine.
But the general question of why run low doses at all, there's a dopaminergic angle most people miss. At high doses 300mg+ test creates a huge surge in dopamine signalling, that's a big part of why a blast feels so good, the incredible sense of well being and drive isn't just the muscle it's the dopamine. However it's a double edged sword. You can't redline your dopaminergic signals and then drop to low doses and expect to feel normal afterwards, your system recalibrates to that high state so low doses feel flat and depleted not because anything's wrong but because your baseline shifted up. Personally anything under 200mg doesn't feel great to me for exactly this reason.
And that's the honest catch, the same mechanism that makes high doses feel amazing is what makes it hard to come back down and what drives the dose creep over time. You're chasing the dopamine.
So for you specifically, a word of caution since you said you're blasting next bulk. Enjoy that 120mg feels good right now because you haven't redlined your dopamine yet. After you blast and come back down this same dose might feel a lot flatter. Not saying don't do it, just go in knowing the "I feel pretty good right here" you've got now is partly because you haven't surged your dopamine with a blast yet, once you do this dose won't hit the same.
Steroids really mess with your brain chemistry so be careful!