A metal stinger is supposed to condense the moisture out of the smoke and in theory help eliminate, or slowdown, the moisture from going up the pipe stem and into your mouth. I’ve had several pipes with stingers in them. Some work better than others, but all of them only work up to a point.

The disadvantage of stingers and also the alternative is that stingers won’t allow you to shove a pipe cleaner into the stem and preferably all the way into the bowl to wick the moisture out that pools up at the bottom the bowl.

I usually pull the stingers out and don't use them. I prefer shoving a pipe cleaner into the stem when necessary. 

I think @Gripsholm covered it, I have always removed them myself. I haven’t had notable benefits from using them. One other thing I would add is that some of them really restrict the airflow more than I like, some prefer their airflow more restricted. Has a lot to do with how you smoke. For example, breath method vs puffer. Overall I would say there’s a valid reason they are really uncommon in pipes these days.