A pretty (common) use case is to have a variable update only upon a condition. For example updating a minLength value:

if minLength > len(cat)- len(dog):
  minLength = len(cat) - len(cat)

Which is inefficient because it has to do the calculation twice, plus you have to type it out. Typically the optimization would be

lenDiff = len(cat) - len(dog):
if minLength > lenDiff:
  minLength = lenDiff

With python’s walrus operator, you can do an assign statement inside an if statement

if minLength > lenDiff:= len(cat) - len(dog):
  minLength = lenDiff