Hello Teachers and fellows.

When I looked up the meaning of "Frugal" on my Oxford dictionary, it shows the following.
"1. Using only as much food or money as is necessary"

The thing made me confused is the linking verb "tobe" in red color.
Could you please explain why it stands there and which grammatical structure is applied.
Personally, I always write " ....as much food or money as necessary" instead.

Thank you.