Banks send bills which are damaged or defaced back to the treasury to be destroyed and get an equal amount of new bills in return. They distribute the new bills. While any individual bill could be traced back to the original bank you got it from, non sequential bills implies they were gathered from various places and have been in circulation long enough that any given bill will have exchanged many hands and thus would be unlikely to be traceable. If you go to a bank and withdraw a bunch of bills, you might receive sequential bills. Showing up to deposit sequential bills which were withdrawn by a single person is solid evidence of you interacting with that person. If they get caught with drugs and their accounts investigated, they might see that they withdrew a bunch of bills that were then deposited by you and come after you