[Haskell-cafe] Empty case alternative

Hello cafe, I was looking into Haskell Report 2010 and notice something in the definition of case statement: https://www.haskell.org/onlinereport/haskell2010/haskellch3.html#x8-460003.1... Apparently an empty alternative within case statement is a plausible production. ('alt' production rule, third option). If I get it correctly, following example shows the empty alternative:

cStmt e = case e of { 'a' -> True ; {-- empty here --} ; 'b' -> False }

What I miss is any ratio behind this. What is the empty alternative good for? Any idea? Franta