A DST is optimal when the variance of the signal you are compressing grows linearly (as opposed to the DCT, which is optimal for uniform variance). That happens

A DST is optimal when the variance of the signal you are compressing grows linearly (as opposed to the DCT, which is optimal for uniform variance). That happens, for example, when you have a prediction on one side (e.g., intra prediction in image compression) and you are coding the prediction residual. The farther you get from the source of your prediction, the less accurate it is likely to be. Block-based motion compensation prediction residuals in video coding are also not uniform: the prediction error is higher at block edges than the block center, so a DST sometimes works better (e.g., when using a transform size smaller than the motion compensation block size). So still useful for compression, just in more specialized circumstances.