Pins are necessary for pinning the pattern to the fabric and holding the seam allowances together when sewing the garment. Pincushions are useful to storing and accessing the pins easily.
There are a variety of pin types, and Dressmaker #17 pins are generally recommended for patternmaking. However, it doesn't really matter which you use, as long as you understand the pros and cons of each.
Dressmaker size #17 Pins have a flat head and the #17 is medium in length - 1 1/16 inches | 27mm long. The flat head means you can press over them with an iron. The flat head also means that thread won't get caught around the head - e.g. if you are doing hand sewing or gathering. One problem with the flat head is that the pins are harder to see.
Glass-headed pins: are heat resistant and will not melt if ironed. As they have a ball-shaped head they are easier to see and remove.
Pearl headed pins: are longer and are usually used with finer fabrics. They are also easy to see and remove due to their round head.
Pins: Should you remove pins or machine sew over them?
To save time in sewing, some people place the pins at right angles to the seam line and sew over them with their sewing machine. While this does save time, there are two possible issues with doing this. You will break needles if/when the machine needle hits the pin. I used to sew over needles for a number of years and only broke three needles, so I used to think breaking needles was worth the time saved (since I didn't break many). However, the other issue is the one that made me stop this practice; the third needle that broke hit me the forehead, which hurt a great deal and left a cut.
I'm lucky it didn't hit me in the eye. I decided it's not worth the risk of sewing over needles, better to take them out as you go along.