Are you talking Recto or Mark series?
With the Mark series it's largely because they have both pre and post EQs. The BMT stack operates before all your distortion, so you're able to fine tune the flub and woofy-ness. As the gain goes up you can bring the base down to keep things tight, or leave move of it in if you're going for a round lead tone. In most other amps they have to choose an amount of bass cut that works for all their sounds, so they probably compromise on the highest gain sound they thing people will go for.
Then the 5-band lets you dial bass back in, and with a little more precision than most other amps BMT after the distortion.