I recently bought a bleed kit (for shimano brakes from shimano) and used it on my hydraulic brakes (successfully as far as I can tell). During cleaning up I noticed the syringe says it was for single use only. This seemed annoying to me at first (produces waste, requires to actually get new ones - shop opening hours / struggle with shipping companies …) so I was wondering what the reasons might be and came up with the following candidates:
They come from the same production line as medical syringes, where they really really shouldn't be used more than once. So the label is on it by default and the production company never bothered to figure out what actually happens upon multiple uses.
The syringe won't be clean after the first usage and the oil damages the plastic over long exposure, so you get traces of whatever comes out of that reaction into the oil, and the syringe won't work properly (piston not tight / piston stuck)
The oil attracts dust and dirt, so after storage I would contaminate my brake with "bike tool drawer dust".
I might not remember if I used a syringe for mineral oil or dot 5.1 / dot 4 and run into danger of mixing the two when using on a different bike, and all just because I tried to safe a few bucks on a new syringe. (or even worse, who says I only use the syringe for brakes? I might mix all sorts of liquids)