Practice tests without preparation do not improve scores. When taking a practice test, students are putting down as much information as possible, rather than remembering the information.
While this is often a helpful trick, reading strategies depend on the type of passage, and strength and style of the reader.
Often people with high scores have no idea how they got them. They are smart enough to just “know” the answers. Preparation skill comes from knowing strategies that work for all students.
Test makers have their own agenda: make the test accurate for their purpose. The best way to do this is to have students answer questions the “typical” way, not the most efficient way. Test preparation, however, teaches students efficient strategies to tackle problems.
Many High Schools offer preparation courses. While it is convenient and often free or at low cost, teachers are not necessarily test preparation specialists. In our experience, these courses are usually "content" review rather than strategic instruction.