The idea behind these is that there may be a benefit in summing up all the evidence from several similar trials, analysing all of it together. This way, as the sample numbers grow, more subtle treatment effects may surface (because smaller trials may have been underpowered and thus many type 2 errors may have been committed).
However, the statistical analysis of the evidence in a meta-analysis of trials can occasionally produce results which contradict the actual trials. One is left wondering: which methodology is flawed? Whose statistics are faulty?