Testing will let you know whether you are taking the right amount of insulin. In most cases, one will test the fasting blood sugar to know if the long-acting or basal insulin dose is correct. If the fasting sugar is too high or too low, the long-acting insulin will need to be increased or reduced.
Short-acting or bolus insulin is usually given before eating. It is important to test both before and about 2 hours after eating. This will let you know that you’ve given the right amount of insulin. If the insulin is properly matched to the food you’ve eaten then there will be only a relatively small rise in your blood sugar level with the meal -- let’s say 40 to 60 points. A lot less and you have likely given too much, because the glucose will likely continue to fall over the next couple of hours and may go much lower than desired; a lot more and you haven’t given enough insulin.
As I so in response to almost all of these kinds of questions, individual numbers are rarely the major concern. The point is to understand your patterns and to make adjustments as needed so that you are constantly striving for better blood sugar control.