If you test blood sugar before/when you start meal you will only getting a postprandial reading, that can be a good indicaton of how much you should eat. Like if your postprandial are high you should avoiding too many carbs.
If you rather want to know how high your blood sugar peak (and how rapid) you have to test 30 min after eating. Helpful if you want to diet and need to know how much different foods increases your blood sugar.
If you want to test how effectly your insulin lowers your blood sugar you have to test 2 hours after eating. Can be a good indication of how well your meds are working.
So there is no singel way of test blood sugar, it all depends on the situation and what kind of information you need.