Tron is one of the worst match-ups for Humans decks, so naturally Humans players would like to have some powerful weapon against them. Fortunately, this time WotC decided to give them what they have wanted for so long.
Lavinia is that perfect counter to Tron decks that can't help themselves but cast Karn Liberated or Ugin, the Spirit Dragon on turn three or four. If you have Lavinia on board, then it won't happen that easily anymore. Of course, they can still play their creature spells, but Tron decks rely mostly on their overpowered Planeswalkers.
The second ability of Lavinia works great against Amulet Titan decks that run Pact of Negation and Summoner's Pact. This card kills two birds with one stone.