Think of it as company's news but just posted on Maps. Here is how it was introduced by
Google:
"Google posts empower the owner of the GBP profile to post content straight to the search engine results pages. Posting on Google Business Profile is for the prospects who are already looking for you, unlike the prominent social media platforms."
From 2021, these posts last for seven days if you haven't assigned a date range for it. And it's free, just like creating your Business profile or adding your company to Google Maps.