You only have to buy one of the two options. Both the subscription and the one-off pack purchase currently give you the same features - see Store help page for more details.
In the future, as we add more features and data, most will automatically be available through the subscription whereas for pack owners, these new features may only be available as an additional pack to purchase.
As developers, we like subscriptions as it gives us a predictable income stream that we can use to make future development decisions. Essentially, it helps the development especially when server and data costs are monthly costs.