The 49ers had a brutal run of injuries to their running backs last year, beginning with the September 1 ACL tear of lead back Jerick McKinnon. Every running back on the roster missed games, creating room for several substitutes— including UDFA Jeff Wilson, Jr., who was promoted to the roster late in the season and succeeded until he, too got hurt.
As a result, the team signed Tevin Coleman to join their returning (and hopefully healed) backs, making their running back room one of the deepest in the NFL — at least if you like dual-threat, fast-more-than-sledgehammery runners suited to this outside zone scheme.