Any seamstress who uses a pattern (and probably even the ones that draft their own patterns, I guess) needs a way to mark the cutting lines and seam allowances on the fabric. Most of us use chalk for variable reasons...
Marking pattern pieces and seam allowances of the pattern on the fabric. Chalk has many advantages: It can be easily brushed or washed out, leaving no trace on the fabric. Chalk is more durable than self removing pen (which vanishes after a week) - if you do not brush over the fabric that much. It comes in a variety of colours (white, yellow for the use on white fabrics, red, blue - I've seen plenty). I use a little tool instead of real chalk because I don't like to sharpen chalk. Actually, cutting or biting into chalk is one of the most horrible feelings in the world for me, which makes me a kind of freak. I don't care. Anyway, my tool comes with refillable chalk powder (can be seen in the little window) and makes really thin lines, very precise. For normal people without chalk issues, a single sharp piece of chalk will absolutely do!
My chalk device has a nifty addition (in the picture on the right) which allows perfect seam allowances as it keeps the chalk line exactly 1,5 cm, 2,5 cm or 5 cm (for hems) away from the pattern. I'd love to have a 1cm option which is the seam allowance I use for jersey, but I'll do with the others as well!
How much is it?
Normal chalk for sewing is a dime a dozen. Dirt cheap. Really! My fancy "I hate to handle chalk because I'm a freak of nature" costs about 10 Euros which it is worth for me, but may not be for you. The chalk refills (also in other colours) are 2 for 4 Euros.
Who needs this?
Anyone needs chalk for sewing! It doesn't have to be my expensive version, though...