Great question!
Before genetic engineering, people who needed insulin to survive (diabetics) had to obtain their insulin from pancreases of pigs! When pigs were slaughtered, they removed the pancreas to take out the insulin. Expensive, time consuming - and for jews, not kosher!
Genetic engineering changed all this - they have cloned the gene for human insulin into a plasmid which they then express in the bacterium Escherichia coli (E. coli). The E. coli makes A LOT of insulin this way - it's made in a stainless steel fermentor (think of how they brew beer) - and it is easily purified. So, now there is human insulin (not pig - so kosher!) and is made fast and cheap!!! No need to slaughter an animal for this anymore!