The mantua was the most formal gown a lady could aspire to and was almost exclusively used at court and for important weddings. Mrs Delaney for example, gives detailed accounts of her contemporaries' mantuas at various royal weddings she attended. They were intended to make a statement. In this case, the bold use of Imperial yellow silk damask adds exoticism to the appearance of this lady, speaking of her ability to acquire silks and luxuries from the farthest corners of the world for her own adornment.
Note on the silk: The Chinese were weaving silks for the European market at this time. However, this damask is not woven with traditional European flowers but woven instead with traditional Chinese Bhuddist motifs and lotus pods.