My company sells these and I have used them in different ways in the past. They seem to work best for liquids that are thicker consistancy. Anything water thin or thinner just pours out no matter how thin the needle. With thicker glues it works well and paints do well too.
You have to experiment to get the hang of it. I have a set on my work bench and have used them for glue application but in the end they are more work then they are worth. Since they are usually not air tight, anything stored in them eventually dries up or gets too thick to squeeze out.
I end up using a fine point brush most of the time. There may be a use for them but so far I have not been able to find it and I get them for free.
We mostly sell them to people using a lot of glue or other liquids where they will be refilling on a daily or semi regular basis. Even we have been told by our customers that once they are done for the day they tend to squeeze any remaining liquid out and clean the bottle and tips otherwise the liquid dries up in the bottle and tip and they become useless.