Possibly an oversimplified reason for justification..
I tend to catch a train to Crofton Park from Central London every day because I can pop into Co-op and Budgens on way home. I live between Crofton Park and HOP so I'd think twice about taking this route in the future. A supermarket in HOP would also mean I'd frequent the Deli there more often too - so the high st would benefit a little ( I stress a little!). Now - what that actually could mean is that my spending would move to HOP and Crofton Park therefore loses out or rather they both get 50% of my business. is that a good thing? I don't know