Paul uses a rhetorical question to introduce the next topic he wants to discuss. It can also be translated as a statement. AT: "I will tell you what the purpose of the law is." or "Let me tell you why God gave the law." (See: [[rc://en/ta/man/translate/figs-rquestion]])
This can be stated in active form. AT: "God issued the law with the help of angels, and a mediator put it into force" (See: [[rc://en/ta/man/translate/figs-activepassive]])
God gave his promise to Abraham without a mediator, but he gave the law to Moses with a mediator. As a result, Paul's readers may have thought that the law somehow made the promise to no effect. Paul is stating what his readers might have thought here, and he will respond to them in the verses that follow.