I figured it was for the job; those guys memorized a bunch of stuff and kept a bunch of their worlds around themselves under their control, but memory isn't perfect, and shtuff happens. If you forget or lose something crucial or find yourself deprived of important possessions by error or bad luck or someone else's malice, it helps to have a backup method of retrieving lost information and/or resources. I, for example, keep my account IDs and passwords and step-by-step directions for certain processes recorded even though I know the stuff, and have been known to keep a truck key on the outside of my truck (hanging from a hard-to-spot hole in the frame on the underside) in case of self-lockout even though I never lock myself out.

But, for them, the backup would have to be something they still have even if they've lost everything else and unlikely to fall into someone else's possession even when everything else has. That essentially means it has to be storable in the body, which then means that it has to be small in scale and scope. And the only way for something that limited to be that useful when you're that desperate is if it gives you access to a very generalized stash of stuff (which you can then use to get whatever more specific stuff you might need in the specific situation).

Another explanation would be that even before the moment we're shown when he changed his mind, there might have been two personalities struggling for dominance, and one of them planted the thing for when it would be able to get rid of the other one, but that goes against too much psychology.

Anyway, I found it a minor enough conceit to enable the story in any case, because the first one had such a great story... sorto like accepting human aliens who speak English because it lets you watch Star Trek. The problem was that the second and third movies haven't had any story to them; he has nothing good to live for or fight for anymore, only bad stuff to take revenge against, and a simple revenge movie is nothing compared to a movie about someone struggling to figure stuff out, start over, and make a new good life for himself. Even with a "happy ending", a revenge story still ends with the original problem not fixed and nothing made any better, so what's the whole point?