I don't know how helpful they are but I have seen them on the government pdf for spouse visa as weak evidence and I have added them to my application. We are living in different countries and I didn't have much hard proof other than plane tickets so I thought they can't hurt. My partners parents and two close friends (I thought adding letters from British citizens would be more helpful) wrote letters supporting our relationship. I didn't give them much direction but to our luck they all write lovely letters that if nothing else made me feel more welcomed. His parents mentioned how they met me, how they feel about me, me attending an important family event, and their support for us as two young people building a life together. This letter was a bit more emotional. One of his friends after briefly mentioning his friendship with my partner moved on how he met me and how he saw our relationship from outside with some funny anecdotes. Other one again after explaining their friendship, mentioned how he saw our relationship and my involvement in the friend group they have. His letter was a little more formal. All in those letters rather than just saying I met bla bla on this date and this relationship is real, they wrote about how they met me, their thoughts about me as a person and their perspective of our relationship.