As you already are aware, rest days are a very important part of training, this is when your body gets to rest and repair itself. In order for the muscles to repair, adequate amount of protein is necessary. Therefore it’s important to look at the macros in one’s diet. You should be meeting your requirements everyday, even on rest days.
In general, most people are not able to meet their daily protein requirements from food alone. Protein shakes make it convenient to achieve your requirements. Also if you have a sweet tooth, protein shakes are a great source in curbing those cravings. Protein shake on rest days is a great way to keep your blood sugar levels steady and will likely leave you feeling more satiated than high-sugar treats.
Another thing to consider is your fitness goals. If you want to lose weight, having a higher protein diet helps you maintain your muscle mass while you are in calorie deficit. At the same time, if you are looking to gain muscle, protein is vital in this. Essentially, the body needs to consume more protein than it uses in order for it to gain muscle. Even if you just want to improve your overall health protein shakes can help you do this. All tissues in the body use protein to build and repair. So even if you want improve your skin, hair or nails, consuming adequate amount of protein is a great way to start.