The main reason is that I've been saving up for this one.
Sometimes you can swear an oath and people may add things or muff it slightly or adjust it, perhaps based on their own religious beliefs or on their own rejections of religious beliefs, whatever the case may be. The oath itself remains absolutely valid, binding in precisely the normal manner.
A really good example of this is the oath that we all swore when we became members of Parliament. Some people have added to that in the past. I remember that when I was first elected, many of us who were Canadian Alliance MPs at the time, added a bit about not just swearing allegiance to the Queen but also to the Constitution and the people of Canada, all of which is irrelevant, from the perspective of the legality of the oath, although obviously of personal importance.
In that spirit, and also in the spirit of religious freedom, openness and acceptance, which of course is a motivating spirit of modern Canada, the purpose of this wording is to make sure that a solemn declaration—which means an oath—remains valid, regardless of whether people add words or use some form of mannerism that is appropriate to them but not part of the formal solemn declaration.
To answer Mr. Graham's question, I think that if I were to add something to the effect of “I'm now going to mess with the system, so ignore everything I said”, that wouldn't count. You're still under oath.
More likely is a situation where someone makes a solemn declaration and feels the need, based on their own profoundly held religious beliefs, to add something indicating their own level of solemnity.