Here's why. Even if you use the same paint from the same original color, the 'touch up' area will have a different coverage of the paint. You most likely will see flashing. This article on Houzz is a good reference.
To use a new can of paint will increase the chance for a slight color difference, not to mention the sheen difference. If at all possible you need to stick with the same type of paint that was used on the wall you are painting. Manufacture's eggshell, satin and semi-gloss differ from each other. A companie's own lines (good, better, best) will differ from each other too. The new paint will look lighter and fresher than the existing paint....especially if it has been up for a few years.
I touched up a wall recently with the original paint. I knew it could potentially create a flash effect but I couldn't take the white spot from a nick created by 'not me' family member A, B, or possibly C ;-) When I look at the wall straight on the difference doesn't show, but when viewed at an angle the touch up area looks flatter than the rest of the wall.
It is possible to touch up with some paints. Benjamin Moore Aura is one of the few paints actually stated by the manufacture that it touches up. Depending how long ago you painted also makes a huge difference of the touch up results. Flat paints flash the least. The higher the sheen the more likely you will have to paint the whole area or get very creative to meld the old and newly painted surface.
Just know that if the person helping you at the store puts up some resistance it's to save you the frustration and to give you the knowledge you may be painting corner to corner, or feathering a LOT more than you planned, not just putting paint on a touch up area.