In BJJ, training the weak side to the point of proficiency can be a very strong strategy.
Most people (even black belts) spending time with their strong side. Hardly anyone is truly ambidextrous. As such, people also spend the majority of their time defending attacks against their opponent’s strong side.
Most people are right handed, which makes left handed attackers difficult to deal with; left-handed people have likely developed strong technique on the opposite side you’re accustomed to dealing with.