Sofas didn't exist before the 17th century. Seat furniture for more than one person was generally confined to benches, which would be pushed against the wall for back support and protection from cold. In the early 1600s, though, an upholstered settee was ordered for Knole, one of the great historic English houses, and became an important prototype for the sofa.
This was around the same time as the development of the Porter's chair or guérite, which was also intended to shield the sitter from cold. Another chair type that was popular for the British elite during the 17th century was the sleeping chair, like the famous 1670 example from Ham House (above), which also featured an adjustable high back with wings, and may have been the antecedent of the wingback chair. The Knole sofa was a close cousin of this kind of chair, and may have developed first.
The original settee was probably ordered for Knole between 1610 and 1620, soon after the estate (which was first built in the 12th century and had once been owned by Henry VIII) had been purchased and renovated by the Sackville family, who still own it.
The built-in back meant that the 'bench' no longer had to be pushed up against the wall, and could be moved out to a more central spot in the room. The characteristic depth of the seat and the high enclosing sides and back may have also lent themselves to a certain kind of behavior — descriptions of Knole-style sofas often suggest that they offered the ladies of the house and their illicit gentleman callers privacy from the servants. Those racy Tudors!
Knole-style sofas have remained popular in traditional interiors. Most antiques that you find on the secondary market are from the late 19th or early 20th century, during the Renaissance Revival. My favorite historic owners of a Knole-style sofa were Laurence Olivier and Vivien Leigh, who were once photographed (above) lounging on theirs with their cat.
Many traditional furniture companies still make Knole-style sofas, including some who mistakenly spell it "Knoll" — Florence Knoll was a 20th-century designer whose own sofas looked nothing like the Knole settee! It's definitely a traditional look, but most new ones are pretty comfortable — do you think you'd ever get one?
Images: 1, 2 & 4 Lost Past Remembered (and #2 annotated by me); 3 the DwellStudio blog; 5 Manolo Home; 6 Emily Evans Eerdmans; 7 Little Augury; 8 Chicago Home & Garden.