The actual reason is to make money of course, everything anyone else says is lies, FUD and disinfo
quark took this to the extreme and it made them millions in less than half a year.
The actual reason is to create a finite supply.
the reason they choke off supply is to raise price for a given demand

of course this greed has gone to the extreme with premine/PoS coins like NXT where they use a block reward that halves every 0 seconds
