Gradation is usually specified for each engineering application it is used for.  For example, foundations might only call for coarse aggregates, and therefore an open gradation is needed.
Sieve analysis determines the particle size distribution of a given soil sample and hence helps in easy identification of a soil's mechanical properties. These mechanical properties determine whether a given soil can support the proposed engineering structure. It also helps determine what modifications can be applied to the soil and the best way to achieve maximum soil strength.