How to mark multiple ins and outs in Adobe Premiere?
In and out markers are used for clip selection, however you have to put the clip somewhere once you select it.  You can either drag or insert it to a sequence or you can drag it to your Project window to have a copy of it saved with that in and out point.  You could later drag from that copy of it in the project to the timeline and it will use the in and out point you had set on it.

The reason you can only select one clip at a time is because the in and out point are designed to be used as part of three point editing.  In three point editing, you specify any 3 of these four points... source in, source out, destination in, destination out.  Whichever one you have the beginning and end of defines the length of the edit and the beginning or end on the other tells it either where to pull the clip from or where to place it.  

This is a traditional way of editing and is how linear systems were used back in the day. The practice carried over to NLEs (non-linear editors) because the process itself is actually pretty quick and easy to use and is what many editors were used to.  It is the least information you need to define an edit.