Pretty much what krsmith said.
Thornton ($8M), Kane ($5.25M), Ward ($3.275M), Hansen ($2M), Fehr ($2M) are all on expiring contracts.
I dunno that the Sharks *need* Tavares, but they could be the only perennial playoff team that has significant cap flexibility this summer. Sharks want to finally get over the hump and win their first Cup? Being able to run Tavares-Pavelski-Couture at center and having Burns anchoring the back end would probably make winning a Cup an inevitability. Don't want to have all three of those guys playing center? Play any one of them on the wing on any given shift. Boom. Matchup nightmare.