You defer judgement because negative feedback of any kind (including facial expressions) kills creativity and ideas. Uttering “crazy” ideas takes courage. You don’t want anyone to feel stupid by having their ideas ridiculed, even if they knew exactly that their suggestion is not feasible. Instead you want others to riff off on crazy ideas. It might just lead to an idea that is radical, yet feasible.

If you assess ideas separately you also increase the likelihood of every idea being considered instead of some being reflexively culled.

You aim for quantity because you’re more likely to get a great idea if you’ve got many to choose from. It also helps turning off the inner judge because if you have to come up with 25 ideas in 5 minutes you’ll have to contribute every idea, including the crazy ones.