The insurance company requires an exam to check on your physical health and determine what type of risk you are for life insurance.
The blood test may check fro several health issues, including nicotine, high cholesterol, HIV/AIDS, diabetes, alcohol abuse, legal or illegal drugs in your system, STD's, etc.
The insurance company reviews the results of your blood test to determine your health, and if they will insure you for life insurance.
If the results of the blood test are contrary to information you provided on the application, they may look at that as you having lied to them about your health, and decide not to insure you, or they may offer you a higher rate for your life insurance coverage.
The information about your health exam is shared with the Medical Information Bureau (M.I.B.) which all insurers can access, so if you apply with other insurers for life insurance in the future, they will have access to this information.
In addition, there is no blood life insurance available which allows you to buy up to $350,000 of life insurance with no blood tests required, if you qualify.