Why blanche vegetables before freezing?
To prevent the vegetable from going 'off' in the freezer.

From answers.com:


  Blanching is the scalding of
  vegetables in boiling water or steam.
  Blanching slows or stops the action of
  enzymes. Up until harvest time,
  enzymes cause vegetables to grow and
  mature. If vegetables are not
  blanched, or blanching is not long
  enough, the enzymes continue to be
  active during frozen storage causing
  off-colours, off-flavours and
  toughening. Blanching time is crucial
  and varies with the vegetable and size
  of the pieces to be frozen.
  Under-blanching speeds up the activity
  of enzymes and is worse than no
  blanching. Over-blanching causes loss
  of flavour, colour, vitamins and
  minerals.