I guess there is a real market niche that ycombinator flushed out-

namely, Impedance matching between -

  a) investors with too much cash and too little time for small startups
 
  b) startups who don't need or want large amounts of cash

You'd think it would be more efficient to cut out the middleman, but I guess the aggregator / 'broker' serves a valuable role, as a retailer.