The Cavs wanted to make the playoffs this year. They figured, not necessarily wrongly, that Irving + other recent draftees + a savvy vet + 1st overall pick was going to be enough to do it. So they signed Jarrett Jack to bring scoring punch and playmaking to the bench and to be a savvy vet.
But it hasn’t worked out at all for Cleveland, and Jarrett Jack has been not as advertised. He was really good last year for Golden State and the year previous for the Hornets. Is the horrible team culture of the Cavaliers to blame? Probably not. More likely, he got paid.