I'm not sure it requires a bank account. People can go by the same stall they bought their sim card from and put money on their account. One of the benefits is that it brings people into the financial system that otherwise don't have access.
People have access to the financial system by virtue of having actual currency.
The question is how you move physical currency out of a payment system, if you don't have actual access to a bank. Essentially if you have "money" in any kind of non-cashable form you can't use it to buy anything. So being paid electronically in an environment where most people you interact with only accept cash/goods is essentially the same as not having money.
The ability to put money into any kind of electronic payment system is, if anything, less important than the ability to withdraw it, especially if you're in an environment that is still dominated by cash payments.
Most cell phone connections in developing world are prepaid. There is a well established system of vendors who accept currency and credit your phone airtime balance. M-Pesa leverages the same system by turning them into banking agents. You can deposit and withdraw funds by visiting your neighborhood grocer.
But does it require a bank account (which was my main concern)? Per http://www.afd.fr/en/developing-mobile-banking-madagascar only 5% of Malagasy had access to a bank account in 2014.