Wikipedia defines snack as a small portion of food that is eaten between meals. The way I think about it, that is the only distinction between a meal and a snack. That "in between meals".

This, as far as weight goes, carries with it an inherent quality that makes regulating weight harder. If not impossible, depending on your sleep patterns (the etymology of the term breakfast indicates exactly how this is relevant to what I am saying here). It's nearly impossible to find snacks that have zero insulin response in your body. Insulin not only promotes energy storage, but it also prevents the body from using energy already stored. Making a habit of doing that, even when you don't face weight problems (which are related to health issues), is essentially making a habit of preventing your metabolism of using energy already stored from previous meals.

This is also probably the most important reason why people speak highly of intermittent fasting or low carb diets. Most of them, through these two approaches, regardless of the other positive/negative aspects, completely eliminate the habit of constantly spiking their insulin levels, effectively allowing the body to regulate energy levels through both the energy still available from a meal and the energy stored from previous meals.