Concept
Director J.J. Abrams went old school during production of The Force Awakens. The original trilogy, which mainly used real sets and props, had a special vibe that was missing in the prequel trilogy. Abrams wanted Episode VII to feel organic and tactile, closer in spirit to the original movies. To achieve that, they used as many practical effects as possible, and BB-8 was no exception.
The first known sketch of BB-8 was revealed during Star Wars Celebration 2015. It certainly wasn't a Ralph McQuarrie illustration, but the concept might be his. In an interview with Star Wars Insider, the artist responsible for the original trilogy concept art, said that he originally envisioned R2-D2 as a rolling droid. According to Episode VII producer Kathleen Kennedy, the film crew looked a lot at early concept art by McQuarrie during pre-production and some unused concepts have made it to The Force Awakens.
The idea of a rolling robot was cool but complex. How do you bring alive a droid like that? CGI would have been the logical choice, but J.J. wanted a real prop. The Pinewood Studios' Creature Shop took on the challenge and eventually succeeded at building and puppeteering BB-8 in the film.