BA, there are many times where herbicides can be benefical to food plots. If you spray before seeding, it kills out existing vegetation making it easier to establish a quality plot. If you spray after germination you can extend the lifespan of a plot by reducing the competition.

According to articles I've read some perennial plots have lasted 5-6 years when grasses were controlled by applying herbicides and adjacent non-treated fields lasted 2-3 years.

That's two good reasons to use herbicides. Not, as you say, "necessary" but they certainly make common sense.