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 a treatment cuts the likelihood of getting an illness in half and the baseline risk of a person getting the illness is 2 in 100, receiving the treatment may be worthwhile, even if it also has harmful side-effects. If, however, the risk of getting the illness is 2 in 10,000, then receiving the treatment 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.