There are three primary reasons.
First, we prioritize individual support requests based on the subscription member's past use of support. I.e. the less support you've used, and the longer you've been a subscriber, the higher you'll be in the queue and the faster the response time you will get. In contrast, newer subscribers and people who open many support requests may have to wait longer for a response.
Second, if you have been a subscription member for a long time but haven't needed much help, we will see that when you eventually do. Under some circumstances, we can use that to your advantage and spend extra time helping you troubleshoot an issue or digging deeper into our system to understand what's going on. When those opportunities arise, the metric we look at is how long you've been a subscriber.
Third is the philosophical reason. Most of the costs of providing you with support have been incurred before you ever seek support. Subscriptions help make sure that when you do seek support, there will be someone there to provide it, and that that person will be someone well-trained and capable of providing the best quality support possible. To put it another way, you probably don't want us to wait until you have a question to go out and recruit, hire, and train someone qualified to answer it.
We acknowledge that we can list as many reasons as we want, but asking you to subscribe to a feature the primary benefit of which is support before you need support is essentially asking for a leap of faith. When we look at a support inquiry and see that somebody made that leap and subscribed months ago, we do everything humanly possible to make them happy they did.