During prophase l of meiosis, crossing over between the sister chromatids occurs. what is the purpose of the process?

Crossing over is a genetic phenomenon that occurs during meiosis and contributes to genetic mixing during reproduction. During cell division, pairs of homologous chromosomes associate, cross and then exchange certain segments of their genetic material, before separating. Some genes migrate from one chromosome to another.

Consequently, each of the gametes derived from the same precursor cell has a combination of genes different from those of the others. Thanks to the genetic mixing created by meiosis and fertilization, every zygote has a unique heritage of paternal and maternal genes (in different degrees). 
Crossing over can lead to an abnormal genome if the exchange is not done equally, which can lead to an in-utero death or congenital diseases.

Ao, bo, ab, bahope these

That is true! many fish species traveled to the u.s. because of it.