Usage in nature
As I recall, two of the sources say the value to the slug is that any predator that eats one of those things regrets it 'cause their jaws get glued shut, and one of the sources says the value to the slug is that the predator can't get the slug off whatever surface it's on.
That's handy, though I was looking for how animals use slugs for healing their wounds. I know they use some plants for healing wounds but using slugs is pretty new.
But then a few months ago we watched a movie by BBC, in which the authors reported usage of a saw by a monkey as an extraordinary example of intelligence of animals that was allegedly not seen before. They gave lots of other clever behaviour examples, such as usage of particular flowers to support their diet, creation of an nicely designed circular underwater nest by a fish, etc etc. While these examples are indeed impressive, I was disappointed by such a saying: birds use sticks for creating their nests for many centuries, and implying that usage of tools is unique to man until BBC showed otherwise sounded absolutely frustrating and misleading.
Yes, the slug uses the glue to secure itself to whatever surface it may be on. One of the sources had a wonderful text-image of a bird trying to pull the slug off a wet leaf but finding itself unable to do so no matter how hard it grabs and pulls. I couldn't figure out how to use it without either plagiarizing or providing information not in the source.
One of the sources did paint a nice word picture of that. Another source (well, two but I think one was getting it from the other) painted a nice word picture of a predator getting its jaw glued shut. When the sources are inconsistent like that, none of it can be treated as "common knowledge"; it's all hearsay. My guess would be that both happen, but of course we don't get to guess and also don't need to.