What purpose (in life) does a slug serve?

They eat decomposing vegetation and animal matter. They also eat other live vegetation. They help in the decomposition process.

Aluminum pie pans full of beer will kill them; simply put the beer in a recessed pie pan (put the lip of the pie pan at soil level, so it's easy for slugs to walk into the beer). They will drown in the beer. Other natural poisons are iron-phosphate based and have the names "Slug-go" and "Escar-GO". They will kill slugs but nothing else (dogs, etc.)

If you'd like a win-win situation, several types of fowl including ducks, geese and turkeys find slugs to be a delicacy and will aggressively search your yard and wipe them out. Unfortunately just about every predator out there from raccoons to foxes and dogs like eating the fowl, so you'll need to protect them.

That many is too many! I agree!

There is a product that you can sprinkle on your yard to kill them.

Or you can sprinkle course salt (not rock salt) before you go to bed, around the bowls and on your patio. In the morning they will be dead. I prolly wouldn't put salt in my yard though.

Slugs do not drown in beer, the salt kills them.

that depends which slug you're talking about.

for example,

ear slug doctor: to make sure you have clean ears

dweeb slug: to eat your tuna mayo

boat slug: to remind you of how much fun rowing can be

I just put salt in them

complex stuff. lookup onto a search engine. it may help!