To ensure correctness in your writing, pencil editing is one of the best things you can do. Pencil editing is a proofreading technique. To do it, touch and speak each word when you edit your documents.
Here's a great example of why pencil editing is so important:
It dseno't mtaetr in waht oerdr the ltteres in a wrod are, the olny iproamtnt tihng is taht the frsit and lsat ltteer be in the rghit pclae. Tihs is bcuseae the huamn mnid deos not raed ervey lteter by istlef, but the wrod as a wlohe.
Were you able to read this paragraph? Did it matter that most of the words in this paragraph were misspelled?
If you don’t touch and speak each word when you pencil edit, you might miss spelling mistakes like these because your mind might think one thing and your eyes might see something else!
PS – University of Cambridge explains the psycholinguistic evidence on scrambled letters in reading in this article.