I wrote my first novel longhand and then transcribed it on to the computer. I'm not sure why I did that. I think at the time, I was intimidated by the big white screen staring up at me and felt more comfortable with a pen. (I hate pencils) Now I wouldn't write a whole book longhand if you paid me. (If you're making offers, though, I'm willing to think it over)
A computer is a marvelous thing. You can cut, copy, paste, delete, take whole scenes and move them around, take Jim and turn him into Scott, all with a few well-placed clicks of the mouse. But there are a few things you should never trust your computer to do for you.
1. Do not trust your computer to know what you're trying to say. I edited a manuscript for a friend and found the word "defiantly" where it should have been "definitely." If both words are spelled correctly, your computer is not going to pick up that subtle difference in a spellcheck. One of the mistakes we make when we're first learning the computer is expecting the computer to find everything, thus eliminating the need for us to do a line edit. Nothing eliminates the need for us to line edit.
2. Do not trust your find/replace feature to change only what you tell it to. I love find/replace. In Strength to Endure, I had to change a horse's name from Featherstep to Amadeus, and later I had to change the horse into a bicycle. I used find/replace in both those instances without a problem. But then I had to change the spelling of a character's name to make it more authentically German. I was spelling Erich as Eric, but a German friend of mine told me that adding the "h" on the end was more proper. So I blithely (and somewhat ignorantly) put "Eric" into the find feature and "Erich" in the replace feature and went on my way. Well, America turned into AmEricha. Frederic turned into FredErich. And I didn't notice it until I was printing it out.
While editing my work in progress, a Vietnam veteran friend told me that in most instances, "weapon" is more suitable than "gun." So I did a find/replace, but then I smarted up. (I do that real good sometimes) I then did a find, and double checked everything that had been replaced. We were getting along just fine until I discovered such things as "weaponfire" and "weapon shots," which obviously should be changed back. And have you ever heard of the color "burweapondy?" I hadn't either. So be prepared to double check and make sure that any odd phrases created by your find/replace are corrected.
In summary, yes, a computer is fantastic. It can shave hours and hours off your total project time. But don't count on it to know what you want all the time and to compensate for it. Use your own eyes to double check everything and don't expect the computer to do the work for you.