Hi

why would one need an attenuator with 34xx version 6?

Sam,

You use one if the included 20:1 attenuators when you expect to encounter more than 50 volts.

50 volts in the max input for the 3000 series.

The attenuator allows you to scale for higher voltages while at the same time protecting the scope from them.

Inductive kicks from coils, injectors, solenoids, etc can reach fairly high voltages.  For example, it is not unusual to see a coil primary inductive kick to go over 400 volts.  The attenuator reduces 400 volts to 20 volts without changing the waveform characteristics.