Google uses your location information to track your locations in order to provide better (more relevant) advertisements through their own ad platform.
Apple does exactly the same but because it doesn't have it's own ad platform this gets sent to their "Approved Partners" who do it on their behalf.
Stopping 3rd parties from being able to use this data simply tightens the circle, the data is anonymous (for the most part) and while there are companies that exist out there to tie this data to an actual person most companies like ours have no need to do so.
Likewise what you will start seeing off the back of this is a decrease in advert quality.
Like anyone - I despise adverts, but if I HAVE to see them I at least want them to be relevant products that I am interested in. Soon the only people who will be able to provide those insights will be Apple Partners or Google.