Drive ratio (ex. 8:1, 12:1) describes the size (circumference) difference between the drive wheel and flyer / whorl / pulley. With each treadle the drive wheel goes around once, but the flyer will rotate a higher number of times. That number depends upon which ratio you are using. So with a 6:1 ratio, the flyer will rotate 6 times to 1 revolution of the drive wheel. This will determine how much twist is put into your yarn per treadle. Lower ratio whorls / pulleys are the larger ones, and will put less twist in. Higher ratio whorls are much smaller and will rotate much faster, thereby putting a lot more twist into your yarn.
Different fibers need different amounts of twist, and therefore different ratios. Thicker and longer fibers, such as Lincoln for example, work much better spun at a lower speed. But then you have the shorter fibers such as Merino and Cashmere need a much higher ratio to get the appropriate amount of twist into the yarn. Otherwise you are going to have to treadle like an Olympic sprinter to get good yarn ;) Changing ratios is a much better choice (and easier on your legs).
A wheel that offers a variety of ratios is definitely the way to go – it gives you more flexibility in your yarn production. Some wheels, however, are just made for certain things. I would love to have a wheel with a 40:1 ++ ratio for my laceweight. One of these days…
The ratio you want to use to produce a given yarns depends on your wheel and your spinning style. Within the general guidelines of higher ratio for shorter / finer fiber, the best thing to do is experiment to see what works for you. If your yarns isn't holding together, try a higher ration. If your yarn is resembling rope or string you probably want to back down a ratio or two. A friend and I both have Schacht wheels, but her standard treadling speed is slower than mine. So to get the same amount of twist per feed (winding yarn onto the bobbin) she either has to treadle more, hold onto her yarn longer, or use a higher ratio whorl / pulley. She chooses the latter ;)
Don’t forget to keep notes of what rations work best for which yarns / fibers. It helps a lot.