I enjoy* DotA2, but I have never gotten into watching competitive play. I don't get any excitement from it. The mechanics of the DotA2 means that if you do well in the early game, it's very hard for the opponents to come back. Even if the opposing team synchronises abilities beautifully and perfectly, the raw numbers of being ahead with levels and items means the other team will probably survive and turn around the initiation. It's not a fun experience seeing one side easily dominate the other, there's rarely any back and forth.
And also, it's hard for me to enjoy watching someone else plat a game I like*, because I could be playing that game I like* instead.
As a result I haven't bought the compendium because it's not what I am interested in at all. I have also never paid money for any skins in DotA2 so that doesn't appeal.
I do hope the people competing are enjoying themselves, because it must be hard keeping up high quality performances in the game. Because your team and any sponsors rely on you maintaining a high standard. There's no room for a relaxed approach.
It's nice that the prize pool is so big (though does it actually need to be?) as it hopefully will help e-sports seem more serious as a sport in the world of Snooker and Darts and Horse Dancing.
*when my enjoyment isn't being compromised by toxic players.
Well, does the winner's prize at the upcoming FIFA world cup need to be over $27m? When all of the players already take such huge salaries? eSports players don't have wages to fall back on (unless you're in Riot's LCS) so tournament winnings are all they have to rely on. And in a field as competitive as Dota 2, and with some nations (like Sweden) gladly taking up to 40% of your tournament earnings as tax, and many team sponsors taking up to 20% as well, winning The International isn't an instant ticket to just live comfortably for the rest of your life, much less the rest of the year. Alliance, who won last year's The International have freely admitted that the money they won there has already run out.
So yeah, to help make being a pro-gamer a doable career path such prize pools help a fair amount.