Currently, the only use I can think of for the sibling position is to enable a formula for automated WBS outline numbering that adjusts when rows are reordered and indented/outdented.

Here is a formula I was able to get to work in SSDC for automated WBS numbering in which the WBS number is in column C and column B is the indent level using the formula =COUNT(ANCESTORCELLS())

IF($B7="","-",IF($B7=1,COUNTIFS(OFFSET($B$6,0,0,ROW()-ROW($B$6),1),1)+1,IF($B7>OFFSET($B7,-1,0),OFFSET($C7,-1,0)&REPT(".1",$B7-OFFSET($B7,-1,0)),LEFT(OFFSET($C7,-1,0),FIND("^",SUBSTITUTE(OFFSET($C7,-1,0),".","^",$B7-1)))&(COUNTIFS(OFFSET($B$6,0,0,ROW()-ROW($B$6),1),$B7,OFFSET($C$6,0,0,ROW()-ROW($C$6),1),LEFT(OFFSET($C7,-1,0),FIND("^",SUBSTITUTE(OFFSET($C7,-1,0),".","^",$B7-1)))&"*")+1))))

Alternatively, a formula like the following would not require an extra indent level helper column, and is much more elegant. It doesnâ€™t require the use of any OFFSET functions.

WBS =IF(ISBLANK(PARENTCELL()),SIBLINGN(),PARENTCELL()&"."&SIBLINGN())

With the features of indenting/outdenting, the hierarchical formulas, and defining task dependencies using related rows, there may be less need for WBS numbering. So, maybe you just wait and see how many people request outline numbering. In Google Sheets and Excel, WBS numbering allows you to create formulas that function like =MIN(DESCENDANTCELLS()), but perhaps the features and formulas in SSDC will make WBS numbering unnecessary.