Why is it important to use a weakened virus in a vaccine?

If you used a full active virus, that defeats the point of a vaccine. You will trigger your immune response, but since you would just be injecting the virus, it will make you sick, and your body will be less effective at fighting the pathogens.

Viruses in vaccines are weakened, so they essentially will not "fight" back. They most importantly just contains the antigen markers so the body can trigger the immune response, quickly eradicate the pathogen, and build up the memory cells to that virus.

Injecting a virus that is not weak will just be the same thing as if you got the virus, you will likely get sick.

b brown mice / white sand dunes