The CP35 is a really versatile module. It has 4 attenuators, 2 fixed voltages (+6V and -6V) and 2 passive multiples. The later can be even used as unity mixers. You can add CV or oscillator signals. For example adding a sin and a saw wave to create new forms. It is even possible to add or subtract a constant voltage to the signal.
I think in combination with other system 55 module a good tool.
Unfortunately, it consumes a lot of rack space.
Is it must have. Maybe not, but it is worth the money.