People who touch-type (type without looking) use these two ridges to determine the position of their fingers on the keyboard. Touch typists put the index finger of their left hand on F and the index finger of their right hand on J before they type so they know where their fingers are at.
It is an old format that dates back to the 1870's. It was created by Christopher Latham Scholes, a newspaper editor who sought to arrange the Sholes and Glidden typewriter keyboard in a way that keys on the typewriter keyboard would not jam. Typewriter letters were arranged on big metal bars that would jam up on early models because they would strike against each other if they were in close proximity to each other.
To that end, Scholes actually arranged the keys based on LESS probable word combinations, not most probable. So, for example, in typing the word "the", although the T, H, and E are fairly close to one another, they are arranged so that on an old typewriter, the metal bars that held the inked letter, that would strike the page, would not smack into one another and cause jams.
The popular theory behind QWERTY is that the English language makes the most use of the letters in the middle of the "home row" and less use of the letters at the ends of the keyboard. That is not exactly true. The wisdom behind the QWERTY keyboard was to actually SEPARATE letters that frequently appear together in words, so that their keys would not hit against each other and cause keys to jam. After many years, people got so used to the QWERTY keyboard that we have yet to change over from it, although superior typing models have been introduced.