That's the way humans or "man" selected them to be: to be used for food in the form of meat and milk, for clothing from the hides, manure to be used on fields and gardens, as well as a useful labour-free tool of keeping the grass down wherever need be without having to turn on the lawn mower or use a scythe.
they give milk
Typically in American English the statement would be "a cow is a useful animal" - the first sound in useful is close enough to a consonant that most individuals will use "a" rather than "an".
we use cow to get milk
it can be turned into a beverage (milk), and whipped into cream for butter, and cheese (i think). Its useful cause its a food/drink source and not only somewhat tasty but also healthy with calcium and other important vitamins/nutrients.
Snakes don't help humans..but they are useful for entertainment..but are dangerous.
Yes.Fungi is useful to us.They are used to make medicines.
Humans.
ox, lizard,cow,...
its not useful for the refrigerator its useful for humans only....
"Cow meat" -- aka Beef -- gets eaten by humans.
Yes.
No. tapeworms are pest to humans.