QB contracts are so expensive that even an extremely marginal QB like Jones got paid huge after one good year. The Falcons just paid a QB firmly in the good but not great tier $45 million a year. Meanwhile, that much money can often pay 3-4 pro bowl caliber players depending on the position. If you have a cheap QB on their rookie contract, you can really afford to build out the rest of your roster to the point where it is a serious advantage, so if you can win with a QB on their rookie contract or 5th year option, it gives you a tremendous advantage assuming you're competent at building the rest of the roster. Most QBs are going to need to actually play for at least 1-2 years before they really start to put it all together anyway, so you have a tight window to start that process and still take advantage of having a young QB. In the case of the Giants where you have Jones already and can't really cut him this year, you can get away with sitting a highly drafted QB for one season but by opening week year 2 he has to be the starter or else you're giving up too many cheap years if he's any good.