If you want to be in the business of offering payment services that accept Bitcoins, then you want to be able to do this underwriting. People will be a lot more willing to accept Bitcoins if you can say 'the value of BTC fluctuates relative to USD, but we guarantee that we will always buy them at this exchange rate. If you use this exchange rate for setting your prices then you will never suffer from these fluctuations'. To be able to do this (and not make huge losses), you need to be able to influence the market price by buying and selling in large quantities and by avoiding selling when the price is too low. Having a large initial stash of Bitcoin helps with this.