Dmitry, these are very particular transactions! I can see them! I can feel them! I NEED that definite article! These are THE effects of those transactions and other events - that you mention. Dear me, I'm getting quite excited here! Actually, you could say it all without the article and it would make sense, but I would get a much more general feeling in my head. Honestly, articles are clever psychological devices. They help us imagine, and specify, a situation.

Is that the weirdest answer ever?

We are not talking about "financial effect" generally, but a clearly identified group of them - "the financial effects of transactions". "the" tells you that the noun is identified, either from context e.g. the moon (we know this is the earth's moon) or from extra words such as a possessive phrase (with "of"), as in this case.