Collaboration. The word alone is enough to bring back the anxiety flashbacks from the high school group projects. You already know that you are not the slacker of the group, but you have to steel yourself to get through the project with that one employee who believes he has all the answers and wildly ignores the expertise brought to the table by his co-workers. You know the one. The mansplainer. What these unfortunate know-it-alls may not realize, is how the practice of condescending and discarding the knowledge of a colleague is, not only annoying but is a detriment to the project as a whole.
Lack of trust between teammates weakens the project. The one doing the explaining assumes a lack of knowledge from someone sitting at the same table. What this means, point-blank is that he does not trust his teammate. Whether on a conscience or un-conscience level, he has forgotten the requirements and years of hard work put in to even get to the point of sitting in that seat, to even be in the same room to take part in the project at hand.
Another drawback to the practice of mansplaining is this. So often, the knowledge given has nothing to do with the topic, but rather a means of the speaker letting everyone around them know just how much he knows. This creates tangents, rabbit trails which must be either untangled or endured. While there are several ways to combat mansplaining within a group environment, but having to remind the speaker of the original topic takes time away from working toward the assigned task.
Having a colleague make assumptions is a great time waster. One of the most effective ways of combating the mansplaining phenomenon is something called Shine theory. One element of this uses the practice of amplification to drive home the points a female colleague is saying. When a woman makes a point, another woman will repeat the point crediting the original while doing so. This strengthens the team and ensures that valid points are acknowledged and women get the recognition they deserve.