When you donate your clothes to the Salvation Army, they end up in Africa. But there are no Salvation Army workers in Tanzania who distribute these clothes to the really poor people who you've seen in pictures half-naked with bloated bellies. Imagine the mountains of clothes that need to be distributed to mountains of people, and how much man-power that would need. It would probably cost more than the clothes themselves. These clothes, are given (or sold, I'm not certain) to merchants to be sold in large markets all over Africa. In Tanzania, these markets are called "mitumba," specifically referring to those of second-hand clothes. The diversity of these clothes is vast, and include church soccer team jerseys. Surprisingly, there are a lot of Korean clothes as well, especially in contrast to Japanese ones. Anyways, while this may seem to be in ethically gray grounds, in my opinion, it is the most efficient way of distribution. It creates jobs, invites efficient entrepreneurs in the market, and in the end, the different quality clothes are bought by people with different levels of demand. It's economics in action.
What this also means is that the textile industry in Tanzania is suffering. Many women in Tanzania wear kanga and kitenge, which are colorfully decorated clothes that they drape around their body, or get tailored. Many women do wear second-hand clothes as well, but it's the men who overwhelmingly do so. With second-hand clothes out-pricing TZ-made clothes, the textile industry is not conspicuous at all.
I'm not in a position to say whether cheap second-hand clothes are good for developing countries or not. That's a bit beyond my pay-grade. But I do think about how cheap the presumably-leftover / slightly-defected / stolen-from-factory clothes in China were, and I contemplate a bit more.