Announcing LET

Interesting; this is what I normally achieve by using a sequence of named formulas.  This has the advantage that, what is in reality a nested formula, appears as a sequence of statements on the worksheet rather than hidden away in name manager.  To be readable it really needs the alt+enter carriage return as SergeiBaklan suggests.  I wonder whether an alternating pattern of separators might help, e.g.

= LET( 
      salesData := Sales[[Region]:[Profit]],
      modifiedSalesData := IF( ISBLANK(salesData), "-", salesData ),
      FILTER( modifiedSalesData, Sales[Rep]=SelectedRep )
  )

Perhaps that is too drastic since both colon and equals have existing meanings.  A less drastic variation might be to alternate comma and semi-colon as if building an array constant (semi-colon and backslash in mainland Europe).

I had a quick practice with the function and came to the conclusion that, for me, the most effective way to build a multi-element formula was to start with a single formula and once that checks out, to name it and echo it

= LET( salesData, Sales[[Region]:[Profit]],
       salesData
   )

so allowing the overall formula to be built and tested one line at a time.