Magnesium Carbonate solubility?

Why is it important for magnesium carbonate to be insoluble in water for it to be used in the experiment where you make magnesium sulphate crystals?

I'll have a go. It often helps to think about the alternative.

So, what if magnesium carbonate was soluble in water?

You will be using dilute sulphuric acid, so this contains a lot of water. Magnesium sulphate is soluble in water (you need to drive off the water to form the crystals), so you would have two sets of soluble products in the water (the sulphate and the carbonate).

Since magnesium carbonate is insoluble, filtering it is an easy way to remove any excess magnesium carbonate; alternatively, since the magnesium carbonate is insoluble, you can easily see when all sulphate has been used up - there will be excess magnesium carbonate that has not reacted.