Judging from Iago's contemptuous description of him in the opening scene of the play, Cassio has little military experience but has a much better education than Iago. Iago sounds like a tough master sergeant with years of battlefield experience who has to take orders from a young second lieutenant fresh out of West Point. Iago calls Cassio "a great arithmetician" and "bookish" and a "countercaster" (bookkeeper). Evidently Othello feels he needs someone with a good education to help with the administrative details he will have to deal with as governor of Cyprus. Othello values Iago but more as a soldier than a desk-man. There is also a good possibility that Cassio has powerful connections in Venice who have used their influence with Othello.