Unlike transistors, Vacuum Tubes exhibit a warm bluesy type of sound when driven hard into the region of distortion.
This is a highly desirable sound and one that is easily achieved by turning the volume control on the Tube amplifier to maximum..... Simple.
The problem is while you get this nice bluesy crunchy type of sound, it is WAY too loud. The cats and dogs run away, your family runs away, the neighbors call the cops on you...
I don't know if you noticed, but 40 watts of tube power is just a huge amount of SOUND. To me, it compares more closely to at least 100-200 watts of transistor power. My 40x12 watt Fender Blues Deluxe puts out more "sound" than some 120 watt big name tranny heads feeding a 4x12 cabinet. If you don't believe this, then you probably don't have a tube amp yet!
Enter the "Attenuator", its only purpose is to allow your tube amp to play loud and crunchy, but dispose of some of that volume/power before the speaker gets to blow your eardrums with it.....