What's the use?
This catalog allows merchants to distribute products from their stores on Google.
It is useful for the LIA program (Local Inventory Ads).
This feed aims to make the link between your products and the different data sources. That is to say, letting know to Google, which products are available on stores amongst the one you sell online.
How to use Google Local Products?
It has to used along with other feeds from the LIA program (Local Inventory Ads).
Its setting is similar to Google Shopping channel.
If you sell both online and in-store the same product then it must match the id of your Google Shopping feed and the itemid of your Google Local products feed.
This feed is working through CSV and has to be registered in your Merchant Center.