Why does the QIAamp DNA Mini Tissue Protocol require both ATL and AL buffer, while the Blood Protocol only uses AL?
FAQ ID -633

Tissue is more difficult to lyse than cells in a blood sample. Therefore an additional incubation step in buffer ATL containing Proteinase K is included in the Tissue Protocol to achieve complete sample lysis. Blood cells will directly lyse by incubation in Buffer AL containing QIAGEN Protease. Buffer AL is required in both protocols to ensure appropriate conditions for DNA binding to the silica membrane. For further details on the protocols, please refer to the QIAamp DNA Mini and QIAamp DNA Blood Mini Kit Handbook.