If I have the working capital, is there any reason not to hire someone sooner to do less valuable tasks?

Brooke just said that if I could currently produce at the highest level for 8 hours a day, then I need to hire someone to take on the less valuable tasks; that strikes me as a lot of time to expect to be able to work at my highest level at my core skills. I believe Cal Newport said to strive for 3-4 hours a day of deep work, and that seemed like something to work up to. Is there any reason I shouldn’t hire people to delegate the less valuable tasks to sooner if I can afford it? Should I really be able to do 8 hours of my most valuable work (deep work) a day? That seems outside the capacity of normal human concentration. Are there any concerns about hiring someone before I can do/systematize the tasks myself would somehow harm the growth of my company or do I need menial tasks perhaps to improve my concentration skills through doing other parts of my business?