Substituting freezer paper for parchment paper
Parchment paper is not paper coated with silicon. It's actually paper that's been run through sulfuric acid, leaving it just a really thick matrix of cellulose. Long story short, extremely thick paper without the wood pulp.

Freezer paper is normal paper with wax applied to one side. I wouldn't use it for anything involving heat, as you'd end up eating the petroleum-enriched waxy bit in your food, and the paper might light on fire.

Parchment is used as a spill-proof surface in baking. Additionally, you can wrap foods in parchment (a technique known as cooking en papillote) and stick them in the oven, which effectively steams them in their own juices, so no flavor is lost. Salmon and vegetables are exceptionally delicious this way.