Oh, maintaining cash flow is easy, because credit cards mean that I will get paid instantly! Except for the fact that credit card companies charge a 3.5% fee on all credit card transactions (ever wonder why so many places don't accept American Express? They charge more than that!), which may seemingly not matter if you're selling cheap plastic tat...until you realize you may have a 10-15% markup on your goods, and that 3.5% will eat significantly into your margins. While you could just tack that 3.5% on as a fee, if you're selling to other businesses, where orders can be more than $10,000, that 3.5% becomes $350, which will add up fast. Therefore, you will have to agree on the purchase order that the customer will pay you in 30 days, 60 days, or some percentage up front plus the rest in 30 days, etc.
Now, imagine that the customer does not pay you in 30 days, but you still have to pay your own suppliers, your employees' biweekly wages, consumables, rent...now how are you going to pay for that?
Sure, you could go to collections, or to small claims court, but that means paying someone to shake down your customer - and that's not cheap. Are you going to spend $1000 to shake down a customer delinquent on $500? Especially if they're a customer you already have an established relationship with? Hell no, but unless you're able to convince some there to pay you, either by refusing to sell them anything else until they pay, or just hoping they're simply having a cashflow problem and you'll get paid once they get it resolved, you may not see that money for months, if not years, and if it's a one-off customer you could possibly never see that money at all.