You need to test before a meal for a couple of reasons:
1. You need to know if you need to include a correction in your insulin dose for your meal if you are on the high side before eating.
2. You need to know what your level is before eating so that you can see the effect of what you have eaten afterwards - if you only tested afterwards it wouldn't really be useful as you might have risen 2 mmol/l or 10 mmol/l, you wouldn't know.
You test after meals to try and determine the peak of your blood sugar levels. Again, you need to know the levels before eating, but then you test at 1 or two hours to see what is the highest your levels reach. Ideally, you would test at both 1 and 2 hours as your levels may still be climbing after 1 hour - this would be shown by an even higher level at 2 hours. If, however, you tested at 2 hours and your level was lower than at 1 hour, you would know that you had passed the peak (usually! 🙂)