1. Meta-analysis is a superior alternative to the narrative review when reviewing past research. At best, a narrative review may be able to inform a conclusion about the direction of an effect. But a meta-analysis will provide you with a point estimate of the effect size and a confidence interval quantifying the precision of the estimate. Meta-analysis will normally permit you to reach a conclusion even when the underlying data come from dissimilar studies reporting conflicting conclusions.
2. A prospective power analysis will help you determine your target sample size, but a power analysis is only as valid as the estimate of the anticipated effect size on which it is based. A meta-analytic review of past research will often be the best way for informing expectations about likely effect sizes.
3. Meta-analysis can be used to test hypotheses that are too big to be tested at the level of an individual study. For example, a meta-analysis may examine the effects of contextual moderators such different research settings. A meta-analysis can thus signal promising directions for further theoretical development.
For more, see The Essential Guide to Effect Sizes.