Aw, there is no better invention than elastic.  It's nice and comfortable when you just want to lounge around or when you feel like maybe you ate a little too much at Thanksgiving dinner.  You'll tend to utilize the elastic waistband when it comes to pajamas, lounge and work out clothes.  The reason is that it serves more towards comfort than for style.  Creating an elastic waistband is actually very simple and this is why it's recommended for a lot of beginners.