Here's why:
* It describes a noun: It tells us something about a noun, like "The benefits of exercise are beneficial"
* It can be modified: We can add words like "very" or "extremely" to enhance its meaning: "The benefits are extremely beneficial."
* It can be used predicatively: It can be used after a linking verb like "be": "This change will be beneficial."
Let me know if you have any other words you'd like to know the part of speech for!