When do you need to apply external chain tighteners?
It is hard to define. The internal chain tightener pads can keep the chains tighten under a certain load on the chains. In case the load is higher, the chains will get losen too frequently.
What circumstances make it a good decision to apply the external chain tigheteners?
Can the external chain tighteners be mounted later on?
Yes. It can be mounted on later as well. Here is a little tutorial on how to apply them: