Documentation tends to be repetitive. We wish we could store terms and sentences as they are translated and then re-use them later in the documentation. Applications are updated regularly.
Translation takes too long.
Much depends on personal preference, required functionality, budget considerations, language needs, and compatibility with systems used by vendors and overseas offices. For instance, Swedish company Kommunicera AB is a long-time user of Trados Workbench. However, the old saying about the training in the end costing as much per person as the program still holds. Therefore it's a very worthwhile investment to pay somebody who's been at it for a while to show you the more intricate details. They are designed to improve the quality and efficiency of the human translation process, not to replace it.