All glass,
no moving parts
A picture window is fixed. With no sash to open, the frame can be slimmer and the glass larger than any operating window of the same size, and with nothing to seal around a moving sash it is also the most airtight window there is.
Most homes pair them with windows that open, such as a large picture window flanked by casements or with an awning below, so you keep the view and still get fresh air.

