A diversified trading strategy
Not only do you want to diversify the portfolios you create with different assets, but you also want to have diversified trading strategies. The reason you might want to use different trading strategies is to capture gains during various types of market conditions. You might consider a trend following strategy, a momentum strategy, and a mean reverting strategy. If you are trading stocks or bonds, you might consider a buy and hold strategy along with a pair trading strategy as well as a technical analysis strategy.
Markets move up, down, and sideways. Nearly 70% of the time, markets are consolidating gains as buyers and sellers jockey for position. When markets are moving sideways, you need a strategy that will generate gains. For example, during the first four months of 2019, the EUR/USD moved in a four big figure range.
A trend following strategy would have a difficult time generating profits during this period. Traders who used mean-reverting strategies might have had a better chance of being profitable. Since nobody knows when the markets will break out or begin to trend, you need to have multiple strategies available to take advantage of each market condition.