#1 To eat the pin
Not many might like this point but it is perhaps the most important reason as to why Sami Zayn was added to the match for the WWE Title. Apart from Kingston and him, the other participant was AJ Styles.
Given Styles’ reputation – combined with the fact that he is the number 1 contender for the WWE Universal Title – it would have been wrong to let AJ Styles take the loss, even if it was against the WWE Champion.
Styles is set to clash against Rollins at Money in the Bank in under 2 weeks, and making him lose to Kingston would have been a blow to his image. As a result, the safe way was to add a third wheel who could take the pin and spare AJ of the embarrassment.
And for all the reasons before this slide, WWE chose Sami Zayn.