Wow, this is old.
Some people don't need to ever bother with fee ordering, but sometimes you do. Especially if any of your calculations involve % percentages.
In the event management area, when you click fee ordering, you will see a drag-n-drop display of all items in that event that could carry a fee or discount. From the base fee, to late fees, to taxes, to your custom created fee fields. This ordering feature allows you full control on WHEN certain fees or discounts are calculated. Now, if everything is a flat $ amount, it shouldn't actually matter. BUT, if you use percentages, it matters a lot.
For example, say you have a member discount that applies a 15% discount. But you also have a $5 late fee... and you have a fee field where registrants can add a $10 lunch to their registration. The base registration is $100. Now, do you want the member discount applied to just the base registration? Then any other fee is full price on top of that? OR do you want everything added up THEN put on the member discount? That makes a difference on the final cost.
In this example, if you provided the member discount only to the base fee, you would drag that member discount block up right after the base fee block so that calculation is ordered to process at that point, before any other possible items. So in this scenario, you have a late registrant that is a member and also wants the lunch. The cost would be $100 ($100 - $15 member discount + $5 late fee + $10 lunch).
Now if you wanted all calculations made before the member discount was applied, you'd drag that member discount down to the bottom to process last... or near last at least. Same scenario, the cost would be $97.75 ($100 + $5 late fee + $10 lunch THEN - 15% member discount of $17.25).
Obviously, in this example, the difference is small, but it IS different. For some people, this fee ordering element is a life saver so they can control and make sure registrants get charged exactly as they want them too... and not lose out on part of their event fees due to a calculation that they can't do anything about!
Just one item in a long list of features that DT Register brings to the table that no other Joomla extension does.