The building blocks of financial products are fairly simple-equity and debt. One can mix and match to get convertible debt, preferred stock, subordinated debt but finally they all boil down to a combination of those. Gone were the days where a transaction between A & B would mean a direct fund transfer from A to B. In today's era of risks, tax, legal structures, contractual protection etc, even simple transactions are routed through SPVs. But where legal vehicles are used to split the cash flows, risks and contracts, there are issues there and structured finance comes in then.
Take the typical project finance example. Lenders would demand inbuilt safeguards of SPVs, holding operating cash flows in trust to repay their dues etc. Also, expropriation scarred MNCs may demand an offshore vehicle to hold the assets with a domestic vehicle typically guaranteeing the debt. One can make the structure as complex as possible mixing and matching LLPs, firms, companies, trusts, JVs, association of persons etc(thereby enriching their lawyers/accountants in the process). And at times, it is commercially justified to become more complex if by that, you can split your assets into different pools(with lower credit risk) and issue debt in tranches(thus expanding your investor base). The reason that infrastructure, pharma and banking companies have such complex structures(and plenty of jurisdictions), is to be able to use the right structure to tap an investor base at the best possible rate.