I don't think everyone should avoid all gluten and dairy if they're not intolerant to them, however a lot of people may not realise they have an intolerance to a food or may develop one over time through eating too much of the same foods. In addition it's easy to get into a food rut where you eat the same foods everyday which may mean you're not taking in the full range of vitamins and minerals.
This is where food rotation comes in. For those of you who did GCSE geography this is a bit like crop rotation where you let a field recover before replanting!
The idea is to avoid a food for a whole week, firstly to give your body a break from that food and secondly to help identify if you may be intolerant to it.
To do this make a list of foods you eat very regularly, starting with the ones you have every day or the foods you crave/feel you couldn't give up. This could be dairy, wheat, rice or any type of grain, nuts a particular type of fruit or vegetable (I definitely OD on tomatoes) meat or fish ... basically any individual food. For one week stop eating the food and monitor how you feel for that week and then also how you feel when you reintroduce the food. Then try for the next food type.
Signs of an intolerance may include withdrawal headaches or cravings for the food, feeling very tired when you reintroduce the food into your diet, losing weight by cutting the food out of your diet (water loss), improvement in skin condition, improvement in mood, reduced bloating and reduced cravings for other foods
If you find you don't react to any foods, then lucky you! but it is still worth rotating your foods to avoid developing intolerances and to make sure you're getting plenty of variety in your diet.