Your body needs rest to run faster!
Hard repetitive training creates microscopic tears in your muscle fibers. If you do not rest or do easy recovery runs, you do not allow these tears to repair.
Your brain has time to chill out and recharge on a rest day.
Generally, 1 rest day and 2 recovery runs should be planned in your 6 day per week running schedule. If you’re running once per day. For some athlete’s a rest day could include a cross training session like yoga or for other’s a complete rest from all exercise.
Make the rest work for you.