Why sieve or sift flour? What difference does it make?

A sieve is a fine wire mesh with a handle used in baking.  It is typically bowl shaped or conical.

Purposes

• Removing lumps from 
• powders used in baking e.g. flour cocoa
• other things that can be pushed through the sieve like raspberries to remove the seeds when making raspberry coulis
• Mixing ingredients together e.g. sifting baking powder and flour together for even distribution
• To add air to a mix e.g. a sponge cake.

Alternative:
In many cases you can use a whisk to add air, mix dry ingredients or remove lumps.

Care:
The main disadvantage is that its something else to wash up afterwards.  Wash it dry it and pop it in the oven for a few minutes to dry off the metal fully as your ovens cooling down after baking.  It will prolong the life of your sieve by not going rusty. (I know this goes without saying, but only put the sieve in the oven if it has no melt-able parts and please remember to use an oven glove to handle it).