Granted, they were rather autonomous (one reason why I know I stayed until the end of my contract) but with money in the bank, the new CEO can simply do things as easily as any of the other startups in the top 50. Before, that money would be going through some other big company approval process -- e.g., if reddit wants to hire away a fabulous programmer, it simply gets done.
If things go well, reddit continues to grow and increase its value, at which point, it could potentially be acquired, IPO [RDIT?], or just become a cash cow for Condé Nast.
If Reddit's going to monetize effectively (and that's a big 'if'), it needs to be operationally independent so it can get creative.
As a stand-alone entity, they won't be subject to those rules, and Condé Nast doesn't have to worry about the rest of its companies saying, "but reddit did it".
"The move comes after Conde talked to several investors about selling off a chunk of the company as part of the spinout;"
Reddit doesn't need more money, it needs to be brave, slap up some real advertising, and start generating real revenue.
A website that does 100m pageviews, but makes $10m/yr profit is better than one that does 500m pageviews but makes no profit.
Note that a deal at that valuation did not go through, and that likely that valuation was an important reason why it didn't.