Once you have done your practice, you will most likely be feeling good. You will be more tuned in and connected with your body and with your mind.
This means that you will be starting your day off with an awareness of how you feel and how you want to feel. With this, you will be more likely to make healthful choices in your eating, sleeping, thinking, and in your speaking because you will have set that tone first thing in your day.
I have personally noticed a dramatic difference in how I treat myself after I started practicing my yoga in the morning. Knowing I have taken the time to care for myself in such an awesome way means that I want to continue that trend for the rest of the day.
Lastly, for me, practicing first thing in the morning when the house is still quiet makes me feel like my mat time is extra sacred. I feel like connecting with my own body/breath/mind/soul all alone in the wee hours of the morning sets my day on a very special note.