One of the reasons you might want shed plans 10X12 would be for an outbuilding you want to build yourself.
People are realizing that doing the work themselves can save them money.
In the case of sheds, it can save you hundreds of dollars.
You can also pick from a larger variety instead of just a couple styles that your stores might carry or have in stock.
Many of the websites will give you detailed instructions, material lists, and diagrams to ensure that you can build it without too many difficulties.
Although prices may vary from site to site, none should be terribly expensive.
Since you will be buying the materials, you will get to pick the quality you want.
This could potentially allow you to get better quality without having to pay extremely high prices.
It is usually much cheaper to buy good building materials than it is to buy a well-made shed that is already put together.
If you get shed plans 10X12 you can pick from hundreds of different plans in quite a few styles.
You also get to finalize the look of the building.
You can accomplish this with paint and trim.
This allows you to incorporate your outbuilding in with the styles and colors of your house if you want to; something not so easily done with an already made building.
I'm not saying that you couldn't, but it is usually easier to start out with a blank canvas than to try to cover something else up.