By default, “Grab” on MacOS saves the file as .tiff. Why? There’s no preferences to change it. Fortunately, there’s a pretty easy way by opening a terminal window and running the command line:
defaults write com.apple.screencapture type png
You need to reboot for it to take affect. You can make other choices: jpg, tiff, etc.
You can also set the “location” and read the current values
defaults read com.apple.screencapture