You will fight like you train, as the saying goes, and there is some truth in it. If you have never tried to apply your martial art against a fully resisting opponent, it is unlikely to work as well as you would hope. Therefore, a practical martial arts curriculum should include a variety of common attacks, drilled with realistic speed and power.
A reasonable conclusion, isn’t it? But a surprisingly popular school of thought goes much further, contending that:
You should always train as if fighting, as this is the only way to improve your fighting ability.
This is nonsense, and every martial artist should understand why.