Like most of the gadgets we use, a drop checker isn't essential. If you are just starting to use pressurized CO2 you are probably very concerned about not killing all of your fish by using too much. So, typically, you will start with a low bubble rate, and, because someone tells you that 2 bubbles per second is about right for your 30 gallon tank, you will not use more than that. But, you may have a small leak in your CO2 system. Any leak at all and it is likely that little or none of the CO2 is going into the water.
After a few weeks of this you will be disappointed because you see no effect, but you are too afraid to raise the bubble rate.
A drop checker, used with 4 dKH water and bromothymol pH reagent, will help you a lot with this situation. You can see by the color of the solution whether you have any CO2 getting into the water, you can raise the bubble rate slightly every few hours and see if it has any effect or if you are getting close to a nice green color of the drop checker. Unless you get a yellow drop checker you are not likely to be harming your fish.
In other words, for a beginner with CO2, this gives you the confidence to adjust your bubble rate to where it actually does the plants some good, without worrying about harming your fish by going too far with it. Once you get a green drop checker, you can observe what your bubble counter looks like, and from then on you can just duplicate that bubble rate by looking at the bubble counter.
A green drop checker doesn't necessarily mean you have the "right" amount of CO2 in the water, so you can find that "right" amount by slowly, a little at a time, increasing the bubble rate, while watching the plants and fish, and over a week or two, find what is the optimum bubble rate for your tank.
Without a drop checker you may never see any positive effect of CO2 on the plants, and you may just give it up as beyond your ablility to manage it. Since drop checkers are cheap, I see it as a very useful gadget.