Relative effects (ratios) alone don’t provide enough information for judging the importance of the difference between the two groups. They also may give the impression that a difference is more important than it actually is.
For example, if an intervention cuts the likelihood of a child having a persistent stammer in half and the baseline risk of persistence is 2 in 100, giving the intervention may be worthwhile, even if it also has a mild negative impact (such as a child becoming more self-conscious). If, however, the risk of persistent stammering is 2 in 10,000, then giving the intervention may not be worthwhile even though the relative effect is the same. The absolute effect of a treatment (the difference) is likely to vary for people at different baseline risk.