Reminder advertising is big business e.g., everyone knows Coke and Pepsi yet look how much they spend on advertising each year to keep their brand in the minds of consumers.
You may know they exist but if travelling, you may not know where they exist and a Maps search ad could be an opportunity for McD to highlight their presence over a Burger King, Wendy's, etc.
Also, I imagine some of these ads will be about more than just identifying a business location and could also promote new products, offers, sales, etc. that the user may be unaware of.