Employing a full-time senior-level designer for a year tends to cost more than £50,000, along with benefits (and it's difficult to find a good one who is available). Moreover, you may not always have adequate tasks to keep them occupied, meaning you'll have to pay for their time even when you can't use it.
By opting for the monthly plan, you can pause and restart your subscription as needed, guaranteeing that you only pay when you have work for them to do.