Oh yes, it's these topics that make me realise how finicky I am about my sewing tools.
For years I used only one, a little grey one that came with my dear old Husqvarna 6570 then one day the little red ball fell off!
It had a beautifully smooth blade that rarely caught or plucked the fabric.
It took me a few years to find another that I was happy with, in fact, I've only found a combination that really does that. The top one is the scalpel type mentioned by Sewingsue I assume. I got it when I got my first overlocker but never really got comfortable with it.
The end of the little brush is smooth and very good for bigger stitches and hand sewing while the two small Bernina ones are my favoured ones - the old discoloured Bernina one has a slightly broader smooth blade (like my original one) but the cutter is blunt so I use the newer one for cutting the thread and the other for pulling threads, great with nylon lycra that plucks so very easily. I enjoy using these so much I now find unpicking really quite therapeutic, including overlocking stitches!