I have always done stacked cakes(with cardboard separating the tiers) with sucess but I have never used any kind of separator plates( to keep the cake stacked or separate the tiers). I was wondering if they make assembly any easier? What is the difference from the decorator's point of view?
TIA!

If separated, it's easier because they're transported separately and assembled on site.

If stacked, it can be easier but it depends on how you normally assemble without plates...

If you try stacking a tier (on its own board) directly on the lower tier's supports, it can be difficult getting it there centered without messing up one or both tiers, and hard to slide into place if adjustments are needed because you risk knocking the supports sideways. But, if you normally stack the tier (on its own board) on top of an additional board that's placed on top of the lower tier's supports (just as you would like any other separator plate), it's easier to place the first half of the above tier onto the board and slide it into place without worrying about messing anything up or displacing the supports. (Hope this makes sense.)