Why do you think geologists are especially interested in contacts (where two different rocks come together) when trying to piece together the history of the Canyon's formation?

yes, to understand how the geologic structure was formed as the age of them.

Explanation:

Geologist are pretty interested in contacts. This a powerful tool to understand how was the process of formation. It is called stratigraphy the area that study how the rocks are connected.

For example if the rocks are formed in an horizontal way, it is known that the oldest layers will be at the bottom, and the newest at the top.

Hope this info is useful.