Unlike the o'clock emojis, the "thirty" part gives it a more relaxed feel. It's well-suited for casual appointments or announcing a short break.

This emoji mimics the hour and minute hands of an analog clock, from before digital clocks became common. Perhaps for that reason, it gives off a somewhat classic vibe. It can also be used to express an "in-between time" or metaphorically show that something is in an intermediate stage.