John Gruber questions the two drag modes in Safari:
I don’t see why the intra-window (left-right) mode even exists. With the inter-window mode, you can do everything: reorder a tab within a window, move a tab to another existing window, or move a tab to its own new window. The intra-window mode only allows one thing: reordering tabs within their current window. I can’t think of a single reason why this mode exists.
Actually, intra-window drag is a nice feature that has been available in FireFox and Opera for quite some time. While it usefulness may, indeed, be somewhat questioned (it allows to reorder tabs only, that's pretty much it), I found not once not twice that putting a few tabs next to each other when running comparison or piling information for a report is very nice.
Inter-window dragging works very well together with "Merge all windows" option — in case you were reckless and allowed a bunch of windows to sprout thanks to target attribute of an a tag. The pattern I use in this case is to first merge them all together, then one of the tabs up-down to create a new "anchor" window, then move other tabs over to that "zone." But what I find very much useful after that is ability to move tabs around rearranging them for better usability.