If there is once choice, doctors prescribe it. If there are two or more very similar choices, doctors prescribe one or the other. Adding competition decreases price, sometimes there may slight differences in side effects. May not be worth of the research itself, but getting into market justifies it.

Example:

Great discovery: PDE5 inhibitor helps with erectile dysfunction. Product: sildenafil (Viagra). After the mechanism was invented, tadalafil (Cialis), and vardenafil (Levitra) were pushed into market and FDA approved. There are roughly different 10 PDE5 inhibitors and the market is saturated.