If you are planning on taking Bitcoins in payment for some real product then you want some assurance that you can sell the Bitcoins for more than the cost of that product. You get this implicitly with established currencies, because they have reasonably stable inflation rates and, most importantly, those inflation rates are usually tied to your local economy and so the value of the currency doesn't alter in terms of what you can buy with it in the short term (if you're hoarding significant amounts of currency, rather than investing it in your business, for example using it to pay your employees or buy stock then you're a currency speculator and so have different requirements). With an immature and volatile currency like Bitcoin, you effectively need to buy Bitcoin futures or have some other entity underwrite your Bitcoins.
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.