Would be a cool feature to quickly add to favorites, often used products, when, for example, you go back to your previous day or if you want to chose something from the current day.
On mobile, you can click on it(in the diary view) and then click the star at the top.
On the web, it is a few more clicks because you have to view it first.
@evan5 Oh, that's nice to know. I don't really like to use mobile though. Hopefully that feature will trickle down to the web app soon.