The reason why this is the first tip is because it is the natural progression for any homebrewer. You typically start with extract for its low start costs, less room for error, and limited time needed. Extract brewing is great for these reasons. However, a negative aspect with brewing with extract is the lack of creative freedom. The varieties of malt extract available is limited. When you make a brew with malt extract as the base of the beer, you are held back to the limits of the malt extract. No matter what other ingredients you add to the brew, the base will play a huge part. Think of it as a pizza, no matter what toppings you put on it, the base will largely determine the outcome of the pizza.
We are not saying that extract is bad in any way. In fact, it is the opposite. Malt extracts are produced by professionals who specialise in the process of turning grain into malt extract. They have control over the science and have taken care of all of the hard work so that you don´t have to. This means that the resulting quality of your brew can be high even though there is limited creative freedom. Even some professional breweries brew with extract. Therefore we would recommend beginners and intermediates to brew with all grain.