There is no difference if the pre-orders are released at the same time. It is mainly for things that are going to be released separately.
Say you spotted something that were to be released a month later. Blitzwing and Springer would come out, and be put in your pile you pay for the figures and postage of those 2 figures. You then have the option of having them shipped then and there.
If you leave them in your pile, when the other thing comes out, it too will be added to the pile. You get charged for the figure and the difference in shipping.
It means you only pay the total shipping for 3 figures in one box. If you had them shipped separately you would pay more as you pay for the 2 voyagers to be sent, and then the 3rd figure sent later.
If you get a big pile, the adjustments to shipping become pretty much nothing. I had a pile of $500 and added a couple of deluxes and only had the shipping adjusted by $5. It eventually got to the point where I would add stuff and it would not charge me for the shipping. I think that pile ended up being $800 with $100 postage.
That pile was done over 6 months.