A good understanding of learner needs can contribute to successful course planning.
For example
On the first day with a new group the teacher gives the proposed syllabus aims for the course, and asks learners to prioritise them in relation to the contexts that they will need to use language in.
In the classroom
Needs analysis is part of building learner awareness and autonomy. Asking learners what they feel they need to practise is a good initial step. As well as providing data, it can encourage them to start thinking about their learning and taking responsibility for it.
Further links:
https://www.teachingenglish.org.uk/article/syllabus-writing
https://www.teachingenglish.org.uk/article/designing-business-english-programmes-1
https://www.teachingenglish.org.uk/article/teaching-mixed-ability-classes-1
https://www.teachingenglish.org.uk/article/teaching-without-a-coursebook