In the description for this feature, it says
Master of Runes: You can invoke each rune you know from your Rune Carver feature twice, rather than once, and you regain all expended uses when you finish a short or long rest.
The first half is obvious, but what is the latter half referring to? All the runes with triggerable features already recharge on a short or long rest.
The second part is clarifying how much you recover during a rest. Without that clarification a player might believe that a short or long rest only recovers one use of the rune instead of two so that you’d need to rest twice to recover both uses.