If enabled image data is saved as layer of PSD file.
ImagingPSDSaveAsLayer = 70;
ImagingExtras (at line 83)
This is required to get proper transparency when opened in Photoshop for images with alpha data (will be opened with one layer, RGB color channels, and transparency). If you don't need this Photoshop compatibility turn this option off as you'll get smaller file (will be opened in PS as background raster with RGBA channels). Default value is True (1).