#2 Less pressure on WWE to create new tag teams
With two separate tag team divisions, the lack of challengers for the champions results in additional pressure on WWE to create new teams. The lack of depth on either show has forced the company to churn out random tandems.
Even Cesaro and Shinsuke Nakamura, although together for months prior, were thrown into the SmackDown tag team title scene due to a lack of challengers for the New Day. Another example is Dolph Ziggler and Robert Roode, who became a team and almost immediately won the RAW tag titles at last year's Clash of Champions.
If WWE unifies the RAW and SmackDown tag team championships, there would not be a shortage of opponents for the Champions. They would have the flexibility to float across brands and stay on whichever one has a challenger ready for them.
Having one set of tag team champions is an extremely convenient prospect for WWE, as they will not be required to fill gaps by throwing a team together.