[cornholio]

>A single error in the very large part of DNA that shouldn't vary per individual

However true, that is irrelevant to genetic diagnostics as they exist today. We have no idea how a random error might impact health aside from very limited known mutations that are sufficiently frequent in the population to enable statistical correlation. We are probably decades away from being able to say, for a random mutation, 'this will lead to a deficiency in the synthesis of protein A which impact the development or working of organ B'. We can't even agree on the proportion of junk DNA.

[PeterisP]

This is helpful if you have rare symptoms with no currently available explanation - if you get a list of the "unusual" mutations that you have, and correlate it with the same data from the few people worldwide that have the same issue, you get a possibility to improve that condition.

I recall seeing cases of rare genetic disorders that have been diagnosed that way, by online communities sharing data.

http://matt.might.net/articles/my-sons-killer/ is one story that counters "this will lead to a deficiency in the synthesis of protein A which impact the development or working of organ B". For many parts of DNA we do know what protein it makes. For many proteins/enzymes/etc we have some idea about their function in the body - and if we have a test subject missing that protein, then the symptoms will be even more indicative about this, even if the population is tiny (1 in this example!) and doesn't allow for any statistical inference.

This means that if we really want to, we can try to find out the likely effect and possible workaround of a particular mutation, even if we currently don't have a ready-made answer for it.