Why is lower yield point preferred over upper yield point?

A few materials start to yield, or flow plastically, at a fairly well-defined stress (upper yield point) that falls rapidly to a lower steady value (lower yield point) as deformation continues. Any increase in the stress beyond the yield point causes greater permanent deformation and eventually fracture. Furthermore upper and lower yield point is observed in low carbon steel. But medium and high carbon steel have abundance amount of carbon so it form Cottrel atmosphere at new dislocation sites quickly and thats the reason there is no Lower yield point when load is applied.Once the dislocation jumped to the new sites there is no Cottrel atmosphere , so slightly lesser stress are required to keep the dislocation moving and thats the reason lower yield point appear.