A weight gainer is packed full of not just protein but also carbohydrate and healthy fats, which are all packed in to a nice big total calorie count. This increased calorie intake helps your body keep a calorie surplus which is exactly what is required if you want to gain weight. A mass gainer will also help keep your energy levels elevated so you can train effectively which will help your put on more quality weight with higher muscle percentage instead of just simply adding on fat tissue.