Made it more clear why Kylo is only reforging his mask now: he was waiting until he destroyed the Resistance, but now that he is about to steal everything from Palpy he feels encouraged enough to do it now. The mask is the symbol of the Ren dynasty.

It was already clear in TRoS that Kylo re-built the mask because he is more confident than in TFA and TLJ, he’s so confident in killing Palpatine and getting Rey to join him. We see him calmly and contently embracing Vader’s mask, as opposed to in The Force Awakens, when he was more anxious and insecure.

In the first movie, he tried to hide behind the mask as much as possible, whereas here in TRoS he took it on and off as he pleased.