Hi, the importance of [CLS] token is not only limited to NSP (Next Sequence Prediction) tasks. As far as I understand its importance and its functioning, you can use it for fine-tuning in other tasks too, because [CLS] token is that special token that attends all other tokens in the sequence so it has a representation explaining the knowledge from the context explained in the sequence.
Extending it to NSP task, it learns the representation through self-attention looking around at all the tokens in the context (from both input pair sequence).