Custom Training Zones give you more control over how you track your training distribution. You can create up to seven HR training zones and five pace training zones. Each zone can also be used as a selection tool within the Pro Map, so you can view additional metrics like your total distance, duration, average pace, and average HR within that zone.
Every runner is different. A tempo pace for one runner may be a race pace for another. Your recovery heart rate might be in someone else's aerobic zone. You need customization. Not to mention, there is just too many pace buckets on the current view. It would be nice to condense them in a way that is meaningful to you.