If you manage to hide your method of encryption, you build yourself a defense of indefinite uncertainty.
Unlike a key-space which may have a large, but finite number of keys to go through, there is no limit to the possible algorithms that would process a plaintext to a ciphertext.
More on Key-Method Obscurity in this 6 min Audio Talk.
You devise an encryption algorithm by extending your imagination from the pool of well known and well studied algorithms. Your adversary shares the same pool, and his imagination might trace yours -- so don't be overconfident that your method will not be readily guessed.
Paper & Pencil Methods
Your secret method may have flaws you have overlooked, while a public method is well studied
Common applications for method obscurity are: