Explain why it is necessary to know blood types before a transfusion?

The immune system attacks the donated blood cells, causing them to burst. This may cause serious symptoms, including kidney failure and shock. Antigens also occur in other parts of blood, including white blood cells, a type of blood cell called platelets, and plasma proteins.
These parts of the blood also cause a similar type of transfusion reaction. Or, antibodies in the transfused blood can bind to the patient's blood cells, causing a reaction.
The most important step in treating a presumed transfusion reaction is to stop the transfusion immediately (saving the remaining blood and IV tubing for testing) and to provide supportive care to the patient. More specific treatments depend on the nature and presumed cause of the transfusion reaction. Most hospitals and medical centers have transfusion reaction protocols, which specify testing of the blood product and patient for hemolysis, bacterial contamination, etc.

Because if you need a blood transfusion and they dont know what kind of blood type you are, and they just decide to give you any kind thats available (and they end up giving you a blood type that isnt the same as yours), then your body could start rejecting the blood, which would end up giving you a bunch of health complications, and if they dont fix the problem in time, it could be fatal.

You have to know because not all blood types are compatible with each other. If the blood types are not the same or compatible then you can not mix them together.

If you are transfused with blood that is not compatible with your type, a serious reaction will occur and this can be life threatening. Your body will attack the foreign blood cells=ot a good thing.

if you get a transfusion and the blood type isnt compatible with yours it can cause a serious reaction and possibly death

Because if it is not your type, your body will see it as a foreign threat, and destroy it.