If you imagine the nipple with no threads. One is straight sided, the other, tapers from wide near the nipple, to slightly narrower at the bottom.
Obviously. The straight sided one, will only go so far into a tapered hole and the tapered one may bottom out before getting enough threads to secure in a straight sided hole.
Here lies the body of dear old Dick.
He went through life with a twisted pr*ck.
And all his life, was a lifelong hunt.
Looking for the girl with the twisted c**t.
When he found her, he dropped down dead.
The one he found had a left hand thread.
See, was relevant!