Slugs and Snails are essentially the same thing and yes a slug is basically just a nail without the shell.
Both Slugs and Snails are part of the same class of creatures called gastropods so they are essentially the same but they are different since one has the shell and one doesn't.
Snails and slugs do have a purpose in life just like all other creatures such as animals, insects and even humans which are actually animals.
Every living thing on this planet does have a purpose in the world even though it seems like it doesn't.
Some humans even think they have no purpose on earth but in reality we do otherwise we would not be born or exist as humans on earth.
Both slugs and snails have a purpose in life and they provide food for all sorts of mammals, birds, slow worms, earthworms, insects and they are part of the natural balance of earth.
When we kill slugs we upset that balance but sometimes we do need to remove slugs from our properties when they are doing more harm than good.
If you need to kill slugs you can pour salt on those slugs and it will kill them shortly after as it dehydrates the slug.