"A la carte" coaching sessions can work as a supportive touch-up in the weeks or months following a more in-depth coaching engagement. They provide an opportunity to reinforce learning from prior coaching work, or to tackle a one-off topic.
Single sessions are also useful while you are still trying to define your goals or make an isolated decision. Then once clear about your target, you can switch to a subscription package at a discount to begin working toward those goals. For example, you may be deciding whether to move on from your current job, and use single sessions periodically while you navigate that decision. Then switch later to a subscription in order to work consistently toward choosing your next career and landing the job.