If you'd want to take profit early or exit early with a loss. You can trade options and don't need to wait till expiration. Selling a call "on the shares or against the shares" is only in your mind and only you know that you'd like to wait till expiration. Other people may be trading them daily or holding for a few days, or exiting at some profit level or loss level, or closing one call and selling another, etc.
You'd lose dividend when not owning the stock that pays dividends. But since you own the shares then this shouldn't apply to you, though if the other party wants your dividend then they can exercise their call early to get your shares. This is also rare but may be a more frequent reason for early exercise. Though not sure whether
@gaussian had other types of risk in mind.