If you don't use a microwave cover it can potentially result in a messier cooking experience and could result in food splatter inside the microwave or even result in uneven heating of the food.
Covering of the food using a microwave cover can help to prevent splattering and allow for more even heat distribution.
It's also safe to microwave food without a microwave cover or food cover although it can result in dried out food and a build up of condensation and some foods may splatter.
Not covering of the food when microwaving can allow more moisture to escape the food and lead to dried out food but it's not unsafe to eat the foods.
I don't always use a microwave food cover but for some foods that are likely to result in splatter I prefer to use a microwave food cover but they are not always necessary.
Foods, especially foods with sauces or high liquid content should ideally be covered during microwaving as they can splatter and create a mess inside your microwave.
Covering of the food when microwaving also helps to trap in steam and aid in more even heat distribution and without a cover, some parts of the microwaved food may heat up faster than others.