The pre-order guarantees (for all manufacturers) exist to allow them to begin manufacturing of their vaccines before their testing could be completed. If it turned/turns out a company's vaccine fails trials they would be out millions of dollars for vaccine doses that will go in the trash. More importantly they have finite manufacturing limits.

If all told the various companies can manufacture a million doses a day it will take a year to manufacture enough vaccines for everyone in the US. By the government pre-paying for the vaccines the manufacturers could spin up the lines when they were in early trial phases. Six months worth of trials means many tens of millions of doses available as soon as the FDA gives approval of the vaccine.