Three very simple use cases off the top of my head:

1) Sample and hold => slew => control CV

Rather than have an immediate jump when the sample and hold is triggered you will get a descent or ascent to the new value over your specified duration.

2) Bass drum => slew => inverter => compressor sidechain

The slew generator here will function as an "envelope follower" for the bass drum. When you invert that and pipe it into your compressor's sidechain input the compressor will make everything running through it quieter allowing your bassdrum (mixed elsewhere I think?) to boom in the mix.

3) Sequencer 1v/oct => slew => oscillator

You can use a slew generator here to create a portamento affect. Some slew generators have different values for up vs down, so you could have your portamento only appear as notes go up or alternatively if they go down, you could have different slew lengths up vs down, or you could dynamically gate and vary the slew length over time to have your portamento constantly changing.

There's way more possibilities than these three, but they're some simple examples I could think of.