There's a great post up at TextSavvy about how students found it easier to recall facts from a complicated reading passage when they were simply given a brief description of what the passage was about and which was something the students knew about. In this way, the students seemed to be able to able to activate their long term memory, thereby decreasing the burden placed on their working memory as they read the complicated passage, with the end result being that they understood the passage better with a little guidance than they did without it.
At least that's my interpretation.
Teacher directed guidance that helps students activate their long term memory facilitates students understand what's being taught since it reduces the burden on student working memory during the learning process.
Chalk another one up to the basic skills model of learning.