2.5D timing belt replacement question…

Hello. I have a question related to replacing the timing belt on a 2.5D. I’ve read the manual AND searched the forum, so… Can anyone explain WHY lining up the flywheel timing is that important IF aligning the various timing marks after removing the timing cover does the same thing? This is especially true if verified by looking into the valve cover to verify the valve position…? It’s almost as if the manual assumes other work will occur so just line up the flywheel as a preliminary? Ideas? My balancer, pulley, or crank have no marks on them so… that’s out too. After 30+ years I don’t find that unusual… few of these are “bespoke” at this point but may be made up of 2-3 engines or parts of them. This particular engine used to be a 2.5TD from a 1989 Italian market 90 but is now turbo-less and in a Santana Ligero here in the US, for instance. I had no room in the bay for the turbo, so…
Anyway, why can’t the various timing marks be lined up, the valve position verified, and the belt replaced? Appreciate the clarification as my engine bay is cramp and aligning the timing marks at the flywheel will involve a mirror, a flashlight, and a 70lb Romanian gymnast.

The santanas are a can of worms with sooo many parts that are nls....for example the blocks, while looking identical, were completely different from the solihull ones.....more santana nightmares. They r such aggrivation!!!