There are three reasons to run deficit research (0% and 100%, nowhere in between).
The first, which is huge in the early game, is that beakers and gold income are rounded down to the nearest full number. 5 commerce per turn at 50% slider? That's 2 beakers and 2 gold per turn; 10 turns later you've generated 20 beakers and 20 gold. 5 commerce per turn alternating 0% and 100%? That's either 5 gold or 5 beakers per turn; 10 turns later you've generated 25 beakers and 25 gold. This rounding becomes a negligible factor later in the game when you've got hundreds of commerce, but it can be a major early-game boost.
The second is that you might get better modifiers if you wait. For example, research goes faster when you know more other civilizations who have the tech. Suppose you're in a 6-player game, spending 10 turns researching a tech. 5 turns in, 3 of your neighbors discover that tech. That means you get a 15% boost to further progress on that tech starting on the 6th turn. If you've been running 50% slider the whole time, you're saving about 7.5% of the tech's cost in beakers. If you've been running 0% slider the first 5 turns, then 100% the next 5, you save 15% of the cost in beakers. Or maybe you're working on building a library, and waiting lets you get that nice library beaker production boost. Stuff like that; in general, research gets more efficient as time passes so it pays to defer research.
The third is that it keeps your options open longer. You learn more as time passes, which helps make more informed tech decisions. Maybe you thought you had a good chance at founding Confucianism for a holy city only to discover halfway through the research that someone else founded it. Maybe you were aiming for a Liberalism beeline to bulb a critical tech, and someone else is simply going to beat you to it. Maybe you thought you were in full isolation with no neighbors, then suddenly discover that your island can see the cultural borders of another city on a neighboring island. Maybe you just got backstabbed by a neighbor's declaration of war. Any of these could cause you to want to reshuffle your priorities... which you can do at no cost if all you've done so far is bank up gold preparing to research. But once you start putting beakers into the tech, switching priorities becomes more painful.