The bond & mutual understanding between fellow parents is a powerful force. Having children presents unique challenges and allows us to truly see what is important to prioritize, as well as provides new sources of pain & frustration. The perspective that Yasopp & Daddy share as fathers transcends other facts & details of any situation. It is sort of like being in a special club.
Although Yasopp doesn’t want to be an active parent, he still bears that burden through life, which is a changing experience. Still, despite his personal attitude towards parenting, he encourages Daddy to spend time with his child. It is natural to appreciate the benefit of active parenting in the life of a young one, even if Yasopp is unable to provide that advantage to his own child.
Last note, we also see that Daddy has his own parental pride in how well Usopp is doing. Holding a bond with Yasopp, he has a sentimental attachment to seeing Yasopp’s child thrive in the world.