The modes aren't all winners, but a few of them are terrific fun to jump into: Hot Bomb tasks you with passing the imminently-exploding bomb to other players by crashing into them, and it's a laugh. Taking place in the Maze Bank Arena, it's been doling out generous double experience/cash rewards since it launched. The recent Arena War update is like GTA Online's version of a themed minigame compilation, focusing on vehicle combat.
From Rockstar's perspective, it probably acts a good incentive to get new and lapsed players to give GTA a try, and for me it's a comfortable way to keep up with the game's major updates without buying Shark Cards.
It's how I was able to easily afford an Arena Workshop and the weapon above, having not picked up the game as much since the After Hours update. GTA's numerous free cash giveaways have given people a nice excuse to jump back into the game across the last year.