Paul uses a rhetorical question to introduce the next topic he wants to discuss. It can also be translated as a statement. Alternate translation: "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. Alternate translation: "God put the law with the help of angels, and a mediator put it into force" (See: [[rc://en/ta/man/translate/figs-activepassive]])