Clarkson hasn't played since 2015-16 due to a career-ending back injury. He has one year remaining on his contract, which carries a $5.25-million cap hit. He can be placed on long-term injured reserve and therefore won't count against the salary cap.
The 35-year-old signed a seven-year, $36.75-million contract with the Leafs as a free agent in the summer of 2013. He turned out to be a massive disappointment, spending only one-and-a-half seasons in blue and white.
In order to get Clarkson off the books, the Leafs traded him to the Columbus Blue Jackets for Nathan Horton, who'd already suffered a career-ending back injury of his own and has been on Toronto's LTIR since the deal.
Clarkson played in just 26 games with Columbus before his playing career came to an end. The Blue Jackets traded him to Vegas along with a first-round pick and a second-rounder in exchange for expansion draft considerations.
The Leafs are currently $734,699 above the $81.5-million cap ceiling, according to CapFriendly, with one unsigned restricted free agent - Mitch Marner. They can build an offseason roster with a projected cap hit as high as $92.05 million, then place Clarkson and Horton's combined cap hit of $10.55 million on LTIR when the season starts in order to become cap compliant, according to CapFriendly.
Having both Horton and Clarkson under contract allows the Leafs to get LTIR relief worth the full value of their combined cap hits. Prior to trading for Clarkson, Toronto would not have been at the cap ceiling and therefore wouldn't have been eligible for full LTIR relief without Marner under contract. This trade gives the club more flexibility in case Marner doesn't sign before the season starts.
As for Sparks, the Leafs reportedly placed him on the trade block in June. He was the AHL's goalie of the year in 2017-18, leading the Toronto Marlies to a Calder Cup title. But the 26-year-old struggled in his first full NHL season, posting a .902 save percentage and a 3.15 goals-against average in 20 games.
After trading Clarkson and re-signing veteran blue-liner Deryk Engelland, the Golden Knights now have $1 million in projected cap space with two unsigned RFAs - forward Nikita Gusev and defenseman Jimmy Schuldt.