This is the 5534th most frequent Russian word.
полезен
Useful.
Here, 'полезен' refers to the benefit or healthfulness of the vegetable.
Этот овощ очень полезен для здоровья.
This vegetable is very healthy.
In this sentence, 'полезно' is used to describe an activity that enhances mental well-being.
Чтение книг полезно для ума.
Reading books is beneficial for the mind.
Here, 'полезна' describes a practice that serves a beneficial purpose.