Sometimes it is important to focus on single users and it can be beneficial to interview them individually.
The big advantage of this approach is that you get some detailed insights from a single person and you have a very personal interview with him/her.
On the other hand, however the feedback you will get is not as wide spread and by multiple users.
If you have a very specific target group like pilots it is advisable to interview multiple pilots individually at the beginning to get deep insights into their workflows. Research in bigger groups often are more superficial.