After taking a sabbatical for all of season 2 and then reappearing in the final two episodes, Cat Grant is once again missing from National City at the start of Supergirl's third season -- and her absence causes a major shift in the power dynamics of the show and the city.
In this case, Cat has taken the job of Press Secretary to the President of the United States (in this reality, Olivia Marsdin, played by Lynda Carter). Earth-38's Speaker of the House appears to be a stand-in for real-world US President Donald Trump, so audiences should likely expect some thinly-veiled political commentary more or less whenever Cat is around this season.
It is not a huge stretch: Cat was already on good terms with President Marsdin when Air Force One was attacked and the pair were nearly killed in the season 2 finale of Supergirl. Eventually they would be saved by the Girl of Steel, but not without Cat learning Marsdin's deepest secret: she is a Durlan shapeshifter, living among humanity in secret.
Another real-world discussion relating to Trump will have a broader impact on this season of Supergirl: Cat Grant put her substantial financial and media holdings into a blind trust, and surrendered control of the media empire she built to the shareholders. Those shareholders, in turn, were ready to sell the company to real estate developer Morgan Edge before Lena Luthor swooped in with a higher offer to save (?) the day.
In the comics, Edge owned GBS, a competing broadcasting company that frequently went head to head with the Daily Planet and even briefly stole Clark Kent away from the newspaper. He was also deeply in bed with Intergang, a metahuman-fueled organized crime group, and was ultimately brought down by an undercover expose written by The Daily Planet's society reporter -- Cat Grant.
Later, Lena's brother Lex would buy the Daily Planet as well (a story that has played out in a number of different DC timelines, with varying results), although as with all things Lex, his motives were somewhat less pure than Lena's.
Supergirl airs Mondays at 8 p.m. ET/PT on The CW.