Join Chris Nodder for an in-depth discussion in this video The local maximum problem, part of UX Design Techniques: Ideation (2014).
So why would you want to create a whole set of ideas when ultimately you're only going to be using one of them? You do this to avoid falling into the local maximum trap. The local maximum is a way of describing your ability to design the best possible product. Imagine that you're exploring a new area. You're trying to climb up the highest hill, but while you're climbing up a hill, you don't know how tall it's going to be. It's not until you've explored several different hills, that you can work out which is the tallest.
Now, imagine that the hills in this description are actually different designs that you could use. You don't know which is the best design, the one at the top of the tallest hill, until you've explored several different hills. It doesn't make sense to just keep climbing up the hill that you're on right now, because who knows whether there is a taller hill, that is a better design, until you've explored several different alternatives. And that's what ideation does. It avoids the local maximum trap by helping you explore several alternative designs before you get too engrossed in any one way of doing things.
Rather than just following one single track, you take a short period of time to explore multiple possibilities. Some may end up not being possible, others may not seem too useful, but a few will give you new ideas that you can integrate into your design process in order to create the best possible solution for your user's needs.