Yes, you need them
They generally come in three different styles: round hole, diamond-shaped hole (often referred to as Japanese style) and ‘French’ or ‘European’ style chisels. Besides choosing the style you prefer, they also come with different spacing between the teeth. A wider gap is usually better suitable for heavier projects like an axe-sheath. The smaller gaps (2.7 – 3.3) is for watchbands and delicate wallets and such.
The three images below show (in order):
French-style stitching iron. I have 3.0 and 3.8.
The ones pictured in the middle are ‘diamond-shaped’ chisels that I use. These ones are 4mm. (the gap between teeth).
Finally, the ones at the bottom (with the red grip) are the same but measure 5mm instead.
The diamond chisels I use are fairly cheap. The French style chisels cost a bit more.
The Sinabroks are the most expensive ones.