Nobody has mentioned that with walk breaks you can maintain a faster pace while running, so you get more practice at a faster pace while keeping a lower average pace and not stressing your body as much. Basically a more informal approach to intervals.
Consider if you wanted to run 20 miles at an overall average pace of 8/mi. You might get more benefit, of a different sort, running blocks of 7:15/mi pace and walking for a minute every few miles. It does make a difference because the body mechanics of running 7:15 pace are different enough from 8:00 pace that at least you're getting yourself more accustomed to the feeling of running that faster pace. I'd argue that you'd be more prepared to do actual speed work than if you did a lot of running at 8/mi.
I say this because I recently went through a few months where I increased my mileage but slowed way down, and recently discovered that I lost speed just from being out of practice. I think I may have been better off keeping the pace up but taking more walk breaks.