What do they do, and do I need to use them?

[Toploader]

On Ford and many other makes there is something called a spindle nut retainer, do I need to use this, and what exactly does it do?.........

[85crownHPP]

I think boxes do use them, if it's referring to the wheel bearing/hub/spindle nut. It's a little 'cage' that fits over the nut that the cotter pin holds in place so the nut won't back off (it's near finger loose). Some cars have a 'castle' made into the nut that serves the same purpose, but I'm pretty sure we have the nut+cage setup.
They are re-usable, so you don't need to add it to a parts list unless it is damaged.

[Toploader]

Thank you sir