The papers going back that far are to determine which synthetic steps are experimentally viable. You can look at a molecule and think how could I make this? Well if I broke this particular bond it would make A and B, which I can order from a supplier. So let's search the literature to see if anybody has successfully carried out a reaction that connects an A-like molecule and a B-like molecule in a way that would yield the kind of bond(s) that connect them in our target. That's a lot of searching because for a typical drug, there are 10s to 100s of bonds and a total synthesis can take upwards of 20-30 steps. Thankfully, making and breaking most of the bonds is just not possible given the fundamental nature of the compounds (more specifically, it would take a ton of energy to put the molecule together this way, and if that much energy was applied, it would react at more reactive sites long before the desired reactivity occured). For others it is less clear, which is why having a comprehensive literature sure is useful. If your database goes all the way back to the 1800s you can say with some confidence that a certain step has never been done before and therefore assume its not possible (perhaps an incorrect assumption, but pharma isn't interested in developing novel types of reaction methodology for a target screen because if it hasn't been done already, it would probably take several years and the chance of success is low. And besides, that's what graduate students are for ;)

So the literature search is to find out what's possible. Among the possibilités, the only ones of interest are those that are not covered in patents, hence the second screen.