Why is the protein precipitation step optional when working with cells?

The miRNeasy Tissue/Cells Advanced Kits are the second generation of the well-established miRNeasy Kits. These improved versions combine a phenol-free advanced chemistry with gDNA Eliminator Columns or gDNA 96 Eliminator Plates for safe and convenient RNA isolation. They enable purification of total RNA, including small RNAs ≥18 nucleotides, from animal tissues and cells, including small samples. The kits use spin columns for low-throughput RNA purification. The miRNeasy Tissue/Cells Advanced Mini Kit can be automated on the QIAcube Connect.