There's an old problem in analytics: understanding "clicks to bricks". That is, how do you know your online ads were affective at getting people to go into a store and purchase things.
There have been many solutions to this over the years but most have been spotty at best. Some of these include sensors, location aware tracking, etc.
The only real solution that has worked for the most part was incentive or point systems. Get a customer to sign up for an account with the store and encourage them to let you know who they are in exchange for points or discounts. Since that affectively gave you a userID for said customer to attach the purchase to you could then link that to your online advertising or any other data you may have.
The issue is that this requires you to incentivise both creating an account AND giving the cashier your account number (usually via a card). There are a lot of costs involved and it's never going to get 100% of your customers.
When you look at stats that show people have a max number of point based systems they will join, it's going to get harder and harder to get people to join your humble system, especially if you are up against giants like Walmart.
Not long ago Amazon announced that they would be offering their "Just Walk Out" technology to anyone who wanted to license it. In short it allows someone to enter a store, grab something off a shelf and walk out without ever waiting in line or using a cashier. They are automatically charged for anything they took off the shelf.
My interest is in the "just walk in" part. Amazon mentions that you are required to use a credit card to enter the store. What's going to happen quickly is that you will be required to have an account with the store if you want to walk in without the hassle of using a credit card and having a deposit taken on it. When you have an account (which will have a linked credit card) you will probably be able to just tap your phone to enter a store (or even just walk in and let your phone tell the store you are there).
The retailer will now have the userID of the person and everything they purchased. From there it is super easy to link it to ads you've shown them and any other data you might have on them.
The incentive is the convenience of the technology. No more discount needed, no more marketing of your points system needed and no more fighting with all the other points systems out there just to link your data.