I used to do these at the end of my chest workout to fully exhaust my pecs and to hit/target the inside of my pecs. It's an isolation exercise that really targets the pecs without the addition of the helper muscles you'd use in a compound exercise such as an incline DB press. Plus it's great motivation if the crossover machine is in front of a mirror and you've got a tank on ;0
Finally it's yet another piece of variety to keep the muscle guessing...
I used in place of dumbbell flyes or the pec-deck machine as I didn't like the feel of those movements as much.
I also like the stretch it gives after a long chest workout day as you release the weight back down.
I would go heavy enough so I would reach exhaustion at about 12 reps and do 3-4 sets. Sometimes in the last set I'd do a drop set... but not sure why... maybe just to get all 12 reps out... but it alwas gave a great pump.
Here's some tips for anyone reading new to cable crossovers:
Keep arms at mid-chest level, any higher and you activate anterior deltoids rather than isolate your pecs.
Slight bend in your arms throughout the movement. Common mistake is "presssing" the weight as you would in a bench press, without the been you activate the anterior delts and tri's. A slight bend in your elbows helps isolate your chest.
Lean approx 30degrees forward to keep from favoring one side over the other / which will minimize pulling your arms downward and missing the full concentration/activation of your pecs.