2. Why use these techniques?

Why rely on imagination and intuition when there are rational, systematic methods?

In fact, these two categories of methods are not opposed to each other. Rational and systematic methods exploit knowledge, while group creativity methods exploit imagination. They are therefore best suited to open, forward-looking subjects requiring a high degree of innovation.

Why look for ideas in a group, and not on your own?

Because a group of people will always come up with more ideas than a single person, because a group of people can better stimulate each other to explore new avenues, to "think outside the box", and because confronting different points of view always brings greater openness and richness to ideas.