These letters from family/friends are not listed as must-haves in the guide. But I know quite a few people, including myself, who submitted them. The more people writing about you as a couple and attesting that you are a true couple and signing their name with contact info, the more likely that it's true!
We asked friends, our landlord, our parents, etc to write letters stating things about us. We did not tell them what to write, so they were all different, but basically they said that they knew we were a loving couple, lived together from x to x, dated since x, did things together, supported our relationship, etc. Then they signed it and listed a phone number.
They are not needed, but are helpful for weaker applications (I sent them because we are common-law, not married, so I thought extra proof would be helpful)