Knowledgeable investors prefer to invest over the long term primarily because a longer investment term reduces the expected variability of annualized return. Variability represents risk, and good investors try hard to avoid risk.
It is very difficult to predict how financial markets will move over a week, a month, or even a year, but markets have histrically been quite predictable in their annualized returns over spans of ten years or more. For instance, stock markets have historically yielded about 11% per year when considered over any ten year period, during the 1900s. However, such a long-term reliability of return has not been evident in the first decade of the 2000s.
It is also true that long term investments are easier to manage, require less brokerage commissions and are taxed preferrentially to short term investments, but these are of much less consequence than reducing the risk associated with variability.
