Buddha is one of the avatArs of VishNu.
It is typical of the Devas to visit VishNu (NArAyaN) during His avatAr-kArya on earth. Some of them are also assigned tasks and roles to make His avatAr mission a success, and some are asked to take birth on earth to assist with the mission.
Bramha-Dev (the creator God BramhA, creator of BramhANDa - Universe) , Shiva, NArad-Muni and other Devas headed by King of Devas i.e. Indra, visited Shri KRshNa several times -- individually and in groups -- when He was on earth in Mathura, GokuL, Dwarka...
At His appearance in Mathura as son of Devaki and Vasudev, the devas sang hymns of praise. Shiva came down to GokuL to pay Baby KRshNa a visit, Indra learned a lesson from KRshNa at Goverdhan, Narad Muni was in constant touch and visited Him in Dwarka palaces, BramhA in Vrindavan and Dwarka, and finally when it was time for KRshNa to wind up His stay here, all devas headed by BramhA and Shiva came. They come to remind VishNu it is time to leave.
RAma was also gently urged to leave -- otherwise , He is unable to leave earth due to His overflowing compassion.