Why do scientists want to cut DNA

This is what i think: they want to cut DNA to either do some tests or genetic engineer them because lets say there is someone who is very sick if they take a part of a DNA fro a healthy person who has properly functioning organs they'll cut part of their DNA run some tests on it then they'll do a thing call genetic engineering where I think they basically copy the DNA. Sometimes they'll cut DNA to find like cures for HIV, Cancer and other Diseases.