Seated anything makes it more strict so usually less weight has to be used compared to doing the same thing standing.
Standing allows for heavier weight to be used.
Both ways have their pros/cons and good use within the weight room.
Standing allows for more weight to be used via small cheating in your form. Not talking about swinging the whole body rediculously either just to lift more weight, but rather I'm talking small amounts of natural body english which can enable one to lift more weight while still being safe about it. Small amounts of cheating are ok and do serve a purpose depending on what your goals are.
On the other hand, doing exercises seated equates to being more strict with certain movements, and can allow one to REALLY feel the intended muscle group working quite a bit better.
Seated movements can also come in handy if you've got an injury to your lower back...where you NEED to take aggravating it out of the equation. Sitting down to do your exercise can allow for this, while still making sure you get a thorough workout.
Sometimes it's just nice to switch things up and do seated exercises from time to time, or
vice versa.