@fBoyle-- Even if it isn't perfectly accurate, we need a basis for making decisions. We can't randomly decide on what to invest in or how to use our resources. We have to have a rough idea of what the results are going to be.
Economic planning is highly important for this reason. Even small businesses ought to have fiscal analysis and planning done once a year. These are usually done periodically (for each quarter) or for the following year. Large companies try to do it for each quarter, but once a year is usually enough for small businesses.
The great part about economic planning is that it makes it possible to compare expectations and plans with the real results. It helps businesses make wiser and wiser decisions in the future.