It could be, for example, if you want to see all the entries that contain breakfast or diner or supper. It would be useful to see them all rather that searching each of them individually.
Just allowing multiple keywords searching would already be a great addition though.
I was using another diary app for years, but been recently hunting for one that has this feature. Since, I am starting to have a lot of entries, it's a bit difficult to quickly search for past ones. I stumbled on yours that I do like a lot in many aspects, but is still missing this feature.