Cooks do not have to remove the gills from portobello mushrooms. Those who choose to remove the gills can use a spoon to scrape them off.
Removing the gills from portobello mushrooms can be beneficial to the presentation of the dish being prepared, because it may turn other ingredients black due to the gills lending their color to everything someone cooks them with. The color discoloration may affect any sauces, stuffings or dressings someone uses in their recipe. Some gills hide a little sand from where the mushrooms grew, so removing these can prevent sand in food. When sand remains in the gills, this can make food gritty because cooking the mushrooms does not eliminate it.