Great question comrade! I always think about this because every socialist or communist economic system ever built has been under extreme economic pressures from the external hegemony of capitalist countries. That’s why I’ll always be an internationalist. I’m gonna read up on banking in China and get back to you!

One of the main purposes of a financial system is to ration funds from lenders to borrowers, thereby allowing these resources to be put to a more productive use to the economy as a whole than they would be stashed away under a mattress. In a capitalist system, a (commercial) bank operates by charging higher interest rates to the borrowers than they pay to those who hold money in their deposits, making a profit on that difference (the banking spread). The mechanism isn't exactly like this on an individual level (the money you borrow doesn't literally come from someone else's deposit) but its more or less true on the aggregate.

You can't really have commercial banking (and especially stock markets or derivatives) without private enterprise, since the whole system revolves around banks and other intermediaries trying to evaluate the future profits of firms, so they can get the biggest return without risk of default from the borrower.

The problem with the logic of the above paragraph is that, as someone on this site already might expect, the credit market doesn't necessarily allocate the investments that would lead to a country's development or increase its standards of living. So what many countries do (and socialist ones did) is to try to "manually" ration funds through the state by financing long term, low-interest loans to sectors of the economy which are compatible with the government economic policy for the following years (or their x-year plan, if you will). These are Development Banks, which often serve to nudge enterprise in the right direction rather than have an explicit profit motive.

This makes sense if you are a socialist party trying to develop the productive forces in a country through market forces, but not if you are in a centrally planned economy like the Soviet Union (in that case, the loans would be subsidies since they would just redistribute government funds for different uses).

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Ok, you have to keep in mind that the field of finance is a broad field. Finance should be about spread risk among a large group of people that are willing to accept such risk. In finance, you have people doing completely different things. Some people actively trade commodities. In some cases, especially for business, this can be useful to reduce risk of fluctuation in commodity prices. In fact, that what the commodities market what originally intend to do. Let's say you a farmer, you have a crop that you have to wait a whole season to grow to even recoup you initial investment. Price in agriculture fluctuates based on demand and supply. By creating a futures contract, a farm can ensure that his produce will be purchased at a reasonable price even when the supply outstrips demand for the harvest. In a socialist system, the state would take up the burden of always purchasing the produce. However, this create friction in the economic system. If the socialist state had mostly digital currency and a robust economy, these losses could easily be absorb. However, the it's harder for small countries to deal with this sort of thing with a planned economy. The ability to prevent inflation, the very efficient re-allocating of resources based on demand and supply, and the ability to trade with other countries are probably essential for a socialist economy to run smoothly.