When needles are manufactured the eye of the needle is punched with a tear drop shaped punch. The punching process results in the top side of the needle having a slightly larger opening than the underside.  If you are having trouble threading the needle you may be using the smaller hole that was created during the punching process.  Turn it over.  

Needles are designed for the technique. If you look at tapestry and crewel needles, you will notice that they become larger at the eye.  This larger eye (hole) is designed to spread the canvas or fabric threads as the needle passes through.  This reduces wear on the thread. It is not a good idea to use too fine a needle because there will be more wear on the thread making it look frayed.

—Courtesy of Jean

A straw needle is required for bullion knots.  Unlike crewel and tapestry needles, a straw needle is the same diameter for its entire length.  When creating bullions the thread is wrapped snuggly around the needle. To create the knot the needle is pulled through the loops.  A crewel, or tapestry, needle would get stuck in the loops.

— Courtesy of Beverly