Imagine the heavy, thick canvas or tarp. I have in my BOB. How you you sew it, repair a tear or hole in it, etc.? A normal needle will simply not do that job.

Sailmakers needles are larger and stronger than most needles. They are meant to be sued to pull waxed twine or some other kind of string through a heavier fabric, like canvas or some other heavier material. If you are going to use one, in most cases, you probably should be thinking about also using a sailmakers palm, because you will need one push it through thick material. And sailmakers needles can take some force.

Also, if you are going to have any line or rope in your supplies, then you should have twine, a sailmakers needle and, probably, a palm. A former Navy Bosun I had for marlinspike classes was known to say "No sailor worth his salt would use a line without a whipping or a backsplice." If you are using a line without a whipping or backsplice on it, you are abusing your line. Soon it will be unravelling, and the end of it will be useless. And if you put a backsplice in any line, then you have also limited its usefulness, because you have increased its diameter without adding any strength. A whipping is preferable to a backsplice for most uses. If my reference to line is at all confusing, read it as saying rope.

How would you repair a seam that split in your pack? I'd use a sailmakers needle, twine and a palm.