Quick answer
Power has weather. The same man is a lion one day and morning dew the next, and both images are about what it is like to live underneath him.
Read both lines
Both similes come from outside the palace, which is the tell: this is written from the position of someone who has to live with the king's moods rather than from the throne. A lion's roar carries and cannot be argued with. Dew is silent, arrives overnight, and is exactly what grass needs. What the pairing captures is the instability of depending on a person's disposition, since dew and roaring are not two policies, they are two weathers from one sky. The book says almost the same thing at 16:14–15. Proverbs is not celebrating this arrangement. It is telling anyone who lives near power what the sky is like.
Related: Proverbs 16:14–15 · Proverbs 20:2 · Proverbs 28:15
New King James Version®, Copyright © 1982, Thomas Nelson. All rights reserved. Full copyright information. Scripture provided by API.Bible.
Go Deeper
If this verse has been used to put weight on you that it was never carrying, ask about it directly.