If messaging fans, planning content and promoting your page take more of your week than making content, an agency can be worth it. If you enjoy running all of it, you may not need one.
Mostly time and consistency: a chat team that never sleeps, a pricing plan, promotion that runs every day and someone watching your account safety.
You already earn and are capped by time. You want to grow but do not want to live in your DMs. You want help with promotion without risking bans.
You are just testing the waters, you post rarely, or you are not comfortable letting anyone else message fans for you.
Ask for the split in writing, ask what they would change on your page first, and make sure you can leave if it is not working.
QUESTIONS
It can be if it frees up your time and grows your page more than the split costs you. Ask what they would change first.
Yes, if the agency works with new creators and you plan to post consistently.