As an example:
I have two audio tracks with strings on them recorded from two hardware synths. I want to lower the gain by 2dB over the last 16 bars of the song.
I select the appropriate clip on each track, select gain from the menu, select 2dB cut and OK. The gain applies, the waveforms redraw. I play back the clips and now have drums!
Another example:
I want to apply a velocity change to two MIDI clips. They might be on the same track or on different tracks, it doesn't seem to matter.
I right click on one of the two selected clips, Process effect, MIDI effects, Uncategorized, Velocity. Apply the effect.
Now I have loads of spurious MIDI events ( mod wheel, notes or any number of other random events ) added to or replacing my original data.