In my experience, most knives do not come with 20* factory bevels. If they say they are, then that typically means they're somewhere between 20* and 30*. I've seen many knives with factory bevels of 90* per side (that means the edge is flat). In cases like this, you're not really rebeveling, since there was never a finished bevel in the first place. However, you do have to cut a new bevel if you want a sharp knife.
Different knives serve different purposes. 20* may not work for the particular use of the knife.
Thinner angles cut better and increase edge holding. Anything above about 20* doesnt really cut the way most people on a knife forum expect.
If sharpening is done free hand with no guides, stone holders, jigs, clamps, etc., many people just sharpen at an angle they can hold consistently. If the knife in question has a different angle, it has to be rebeveled or you have to change your technique. Its often easier to rebevel.
If you need to sharpen using some sort of guided system, the stated angle, if any, almost never matches the angle of whatever system you want to use. For the system to work, rebeveling is a must, either to a lower angle so the stones actually hit the edge, or to a higher angle. In the case of rebeveling to a higher angle, repeated sharpening will eventually bring the edge angle up to match the sharpening angle.
In terms of resharpening time, consistency is very important. If all your knives are sharpened at the same angle, there is no need to take time to try to match each knife individually. The same technique is used for all the knives and things go faster that way.
The factory bevel may be too thin and become damaged. Rebeveling may be required to increase the strength of the edge and prevent chipping, denting, rolling, etc.
Most books and references on sharpening have the stance that 99% of factory knives have bevels that are thicker than needed, and that the bevels can be thinned and cutting ability increased substantially without the knife taking damage during routine cutting.
) so I see no need for angles greater than 15 degrees.