Architecture History of Domestic Architecture Why does the speaker say this:

In New England today, around eighty houses that were built in the seventeenth century are still standing. Many of these houses—and the documents relating to them—have been thoroughly studied, and some of the houses have been carefully restored to their original condition. These early houses of New England are the greatest single source of knowledge about domestic architecture of the seventeenth century, particularly about details of plan and construction.
All seventeenth-century houses were not alike. There was some variety in their floor plans, with each house being shaped by the circumstances of the family and the characteristics of the site. Nevertheless, it’s possible to identify the three most common plan types, which were the one-room plan, the two- room plan, and the added lean-to plan. We can even figure out the order in which they probably evolved.
The one-room plan was the simplest and the earliest design. It was used in the early cottages at Plymouth and Salem, dating back to the 1620s, and it remained common in smaller and poorer dwellings throughout the century. In the one-room plan, the front door opened into a small entry room, which was then called the "porch." In the porch, there was a steep staircase built up against a massive chimney. The large main room was a combination living-dining-cooking room called the "hall." In this room, there was a huge fireplace set into the chimney mass. The staircase in the porch led to one large sleeping room upstairs.
The second house design was the two-room plan, which was simply the one-room plan with a parlor added at the other side of the chimney and porch. The result was a house with two fireplaces, one in the hall and one in the parlor, that were set back-to-back in the central chimney structure. In several earlier examples of this design, the parlor was actually built onto an older one-room house, enlarging the smaller house. But more common in later examples, both rooms were built at the same time when families could afford it. Upstairs, there were two sleeping rooms, called the "hall chamber" and the "parlor chamber," after the room below each.
The third design, the added lean-to plan, was the result of an addition at the back of the house, making this the largest of the three plans. The lean-to addition had roof rafters leaning against the second-story wall of the main house. The added space was used as a kitchen. The cooking was done in a fireplace added to the back of the central chimney structure. There were two more rooms built on either side of the kitchen. On the cold side of the kitchen, there was a pantry for food storage, and on the warm side, facing the sun on the south, there was a bedroom. Above the kitchen, under the lean-to roof, there was attic space for storage or more sleeping rooms, which you reached by a staircase leading up from the kitchen.
These three house plans form a logical evolutionary sequence. The one room plan was the earliest design. Then the two-room plan was most common up until around 1650. By the year 1700, the lean-to plan was dominant. However, it’s important to note that the one-room plan—although it came earliest—continued to be built throughout the seventeenth century. So, it’s best not to try to determine the age of a colonial house strictly by its plan type, which is not a perfect indicator of the house’s age.
According to the speaker, why are seventeenth-century houses significant to architectural historians?