What does the update feed button do for the Google Shopping Feed app by AdNabu for Shopify?

Do I have to update my feed every time manually using the Update feed button in the Google shopping feed app?

Whenever there is any major change made to the feed,  in order to make sure the changes are pushed to the Merchant Center, you have press the Update Feed Button.

It is not necessary to update feed every time. AdNabu automatically pushes all the changes made every 24 hours. In case you need to push the changes immediately you need the update feed.