Topics Google suggests for posts include: offers, product updates, what's new and events. Did I mention it's free?! Posting on Google is a little bit like posting on other social media, except it is only visible for 7 days and it can pop up where other social media doesn't.
Here is quote from Google Support on how posts work (https://bit.ly/399DZ9E):
How it works
Published posts appear in your knowledge panel when users search for you or your entities. Up to 8 of your most recent posts appear when users search for you on Google. The posts appear from newest to oldest. Your post most likely appears when a user searches for you or the entity you represent. However, We cannot guarantee that your posts show up for any particular search query.
Your posts may show in the Google feed to users who have shown interest in you.
- Views from the feed that your posts receive are included in your metrics
- Event and poll posts do not show in the feed
- Article and story posts only appear if they have an image or video