If you ever purchase the Legendary Bundle, you will lock in a perpetual 15% discount for every other sourcebook or adventure that you unlock in the future. For example, you buy the Legendary Bundle and get all available products at that time. When a new release comes out, you can unlock it for 15% off the standard price, and on and on for all new releases.
The Legendary Bundle will always be updated to include ALL content. That means that buying in at any point has the same effect. When new releases come out, the Legendary Bundle will increase in price to include the new release. If you ever buy the bundle, you receive the benefits.