Why wouldn't you just hire a full-time designer?
That's a good question, and you totally could.
Besides the finding a perfect candidate routine, the starting salaries for a senior-level designer start at around $70,000 - $80,000 and can reach $100,000 per year and sometimes even more.
You might also be stuck with them having low amount of work and paying for their time that you won't be able to utilize with little value in return.
With the design subscription plan you'll be able to pause and resume the service, as often as you need, whenever you have the design work coming up.