Bedrock is the only material in the game that cannot be mined. It forms the boundaries, bottom, and sides of the map. It can be placed and removed in Creative mode, but it can't be removed by mining in Survival mode. This material is a "protection" for evict falling into "the void".
Bedrock is sometimes dropped by Nethereyes when killed. The bedrock collected can serve as a decorative element. Also, you can make such a house (using bedrock) that is unbreakable. This is good as no mob can break the house. It is bad because one placed, you can never mine bedrock in survival mode.
Why it is there
You all should know by now that bedrock is not a troll but a safety measure. It's sole purpose is to make you not fall into the void. Do not think that it is bad but good and useful to trap something like a Pig so you can have a pet forever :D