Here are three reasons why you should ALWAYS report your estimates of the effect size:

1. Your estimate of the effect size constitutes your study’s evidence. A p value might tell you the direction of an effect, but only the estimate will tell you how big it is.
2. Reporting the effect size facilitates the interpretation of the substantive significance of a result. Without an estimate of the effect size, no meaningful interpretation can take place.
3. Effect sizes can be used to quantitatively compare the results of studies done in different settings.