A sefer torah consists of 304,805 letters and each one of them has to be properly formed and complete and, it goes without saying, in the right place!
There are many kinds of errors that a scribe can make when writing even when paying the most care and attention that he can. A slip of concentration however can result in a serious mistake.
To ensure that a scroll is kasher (valid) then, one cannot rely entirely on the sofer. Instead the scroll needs to be proof-read by a magiah (a checker). Indeed there should be three independent human checks, though since the advent of computerised checking (bedikat machshev) this isn't always followed.
Having secured a colleague of mine a commission for a new torah, I found myself in the role of magiah for the first time. In the past I have corrected a scroll which had had computer checks and to a certain extent I am performing as a magiah when I do a restoration check as I will pick up bad letter forms, letter breaks, spacing issues (too much between letters making something k'shtey milim (appearing like two word) or too little between words making it appear as k'mila achat (as one word)) and the occasional spelling error - but it isn't a full proof-read.