When Jesus said that He saw their faith, it tells
us that faith can be seen (by our actions and
words). There is such a thing as group faith in
which the faith of many combined will make
manifested the thing desired.

The very opposite was found in Nazareth when
Jesus was there. He could do no mighty work
there because of their unbelief. (Mark 6:5)

Jesus, instead of rebuking them for tearing up
the roof was pleased with their faith.

The words their faith is important because we
find that when the person needing healing is
unable to believe for themselves others can
supply the faith needed to get them well.