Why? The reasons are twofold;
1.) /a/ would simply be boring and useless otherwise. "anime and manga" is rarely discussed despite the name, spamming of memes or pedo pictures runs wild instead. Absence of "quality posters" (another myth which I've still investigating) and unwillingness of anonymous posters to make /a/ a good board means 4chan has to do the work instead.
2.) Profit. Trolled threads are linked across the internets on various irc channels and websites for "lulz", leading to increase throughput and traffic to the site. People come for specific threads but stay and often browse the other boards even if briefly, and will eventually click on the banner ads, leading to inevitable profit for 4chan. While you argue that this goes into bandwidth for the site and the servers aren't cheap, this is another falsehood. Using his political connections thanks to the success of 4chan in it's early days (particularly the "donate to 4chan" campaign), moot has hosted 4chan at almost no expensive to himself, leading to pure profit on any advertising revenue that comes into the site. Again given the large amounts of visitors and clickthroughs, this leads to a massive amount of money. The current donations don't hurt either.
Now that I've revealed the truth, you'll have to excuse me while I question the true motives of aglets.