It gives you an extra opportunity to spend a ki point for one extra martial arts die's worth of damage (potentially ranged) per turn
You seem to have two major concerns:
1d6 is rather little damage compared to the other available ways to spend a ki point
The damage potential of Flurry of blows is no doubt a lot higher, and the way it is written it clearly applies to only a single attack. However, Deft strikes' trigger is when you hit with a kensei weapon, which means there are some trade-offs that can potentially make Deft strikes situationally good:
- The extra damage is guaranteed
- This can be used after you land a critical hit, which would double the dice
- You can deal the damage at range, if the weapon you're using is ranged
- Extra damage is extra damage. If your target needs to go down now, this might make a difference
White the 1d6 becomes up to a 1d12 later on, it obviously does so too for the extra attacks from Flurry of blows, so that's a moot point.
You can use both Flurry of blows and Deft strikes in the same turn
Flurry of blows allows you to spend one ki point to make two unarmed strikes as a bonus action. The trigger for it is: "after you take the Attack action on your turn". Nothing barrs you from still doing so after using Deft strikes.
