I might be a bit spleen-y, but I have different pins for different tasks. They rest altogether in a bowl (or in a wrist pincushion), but I search for the correct needle while working...
What is it good for?
Well, to pin things together. Either fabric and sewing patterns - if I'm not using my fabric weights which are not good for patterns with small prickly parts - or fabric on fabric or fabric and decorations such as lace... There are a huge number of things to pin for every sewing person* so everyone should have some pins ready!
Most of the time I use pins with a wax pearl head (as above). The ones I own are rather thin but very pointy, so they glide into most fabrics with easy and leave small holes, if any. Unfortunately, you can't iron without pulling these bastards - trust me, I tried. And ruined a nice iron with it! Also they are prone to bending which is a reason that my stash sometimes runs low when I get rid of the curvy ones!
Otherwise I have pins without big heads which are full metal (oh so trve! ^^). I'm not so keen on using them as I forget these in the garments way to often. And it hurts to get stung by an unexpected needle when wearing a shirt or pants! But you can iron over them like nobodies business and I think they last longer without bending - might be me using them less, though...
I also own needle which combine the good characteristics of both other needles: Pins with a glass head! They are a teeny bit more expensive than the wax head ones, but you can iron them and the colour full heads avoid pinning myself because of forgotten needles! Also my needles have a morbid past - they were bought for my anatomy course were they were used to pin dead animals for drawing their insides. Doesn't get much more goth than that, right? **
How much does it cost?
Almost nothing, you can get 50-100 in 1 Euro stores, even fancy ones with heart shaped heads are 1 Euro for 20 pieces there. Watch out, they mostly sell wax ones there, though! The glass head ones are a bit more expensive, I bought 100 for 3 Euros. Still not expensive, right?!
Who needs it?
Everyone who sews!
* I noticed that I use the word seamstress way to often. Which is mean for all the male sewers all around the globe. Hey, equality works in both directions, so I'll try to get better! :)
** For everyone who has hygienic (or other) afterthoughts: My pins have been autoclaved and washed since then, they are probably cleaner now than they were fresh from the store! ^^