Valanciunas was a no-brainer; he's their franchise center of the future. This is a "let's put off having to put in the big extension for as long as possible" move more than anything, though it will be good to get two full years of the young big as a starter to evaluate him. Ross is the more interesting case.
He's scheduled to make a reported $2.7 million this season. The Raptors have Rudy Gay who is likely to opt out this summer and bring in another high-salary, long-term deal, and they have DeMar DeRozan with $38 million left on his contract. So one of them will be moved. Locking in Ross for another year makes him a slightly more valuable asset (cheap contract, multiple years) and gives them stability in case they move DeRozan.
You might even say the move keeps their... options open.