Even if you're friends with another alliance you may want to formalize an agreement that specifies what's to be done in specific circumstances. It helps to be prepared. However, few recent treaties are overly specific, including the ones that I've written, so it probably comes down to tradition. People find comfort in the fact that they have a binding agreement with their "friend" so that they know they'll always have their backs. Of course, as you said, this "promise" is becoming less and less viable especially with people starting to sign treaties more for political gain and less to formalize a friendship. So yeah, it's probably mostly that people are set in their ways and a trend is not so easily done away with once it's started, even if there's really no need for it.
For me, I think it's nice to formally announce a friendly relationship with another alliance and a treaty is a good venue for doing so. The specific contents or wording of the treaty mean little to me beyond being amusing for whoever is reading it.