Grosh uses the location of the user to detect when you're at a store, to sort the shopping list correctly, and to tag the shopping trip with the correct store.
Grosh also uses the location to show nearby offers. If you've given Grosh permission to access your location when you're using the app, this will work automatically. A menu item called Offers will appear in the menu and show available any local offers.
If you're using a web browser, you have to give permission to the website to access your location. In Chrome, tap the "lock" icon left of the website address to show settings for the site. Allow access to location for the website.