The permanent collection is for everything you want to save long-term and that you can share with others. You can create as many of them as you want.
A temporary collection is a place where you can simply dump some pages. That gives you the possibility to inspect them further. The ones you then want to keep can then get promoted to the permanent collection and all other ones can simply get trashed. Each user has only one of it and it can not be shared. The collection which is currently active while being in the temporary one decides where entry get promoted to.