The fourth quarter of 2014 saw the MSCI Japan Index fall approximately 3% (see security EWJ in the graph below). At the same time, the Russell Midcap Value Index rose approximately 6 1/2% (see security IWS in the graph below).
By taking some profits from the out-performers like IWS off the table, and allocating those funds to under-performing sectors like EWJ, one is able to maintain diversification and balance within the portfolio. It also forces one to "sell high/buy low" in relative performance.
Take a look at what happened to the same securities in the following quarter. EWJ rose over 11% while IWS rose almost 2%. In other words, EWJ outperformed IWS significantly in the first quarter of 2015.
See the graph below for the first quarter 2015 performance of these two securities.
By taking some profits off the table and allocating them towards underperforming sectors, an investor could have achieved better yields than if he or she simply held the securities. Also, by rebalancing, one maintains the originally intended asset allocation one desires. If a person wants 60% domestic equities and 40% international, and if domestic equities fall while international rise, then one's asset allocation towards international has become overweighted. Most people do not want an "all the eggs in one basket" type scenario in their portfolio.
Good money managers watch asset allocation and rebalance when necessary. Those managing their own portfolios can do it too. Keep an eye on ratios within your portfolio and don't get stuck overweighted in one security or one sector.