Many minds, better problem-solving
The greater the number of people who can work on a problem, the higher the probability of an optimal solution.
A larger group of people inevitably allows you to access a broader range of different experiences and increases the "human compute time" the group can use to figure out a solution. It also reduces complexity, as transparency in problem-solving requires you to think and state your problems in clear terms.
No matter how smart you are, you cannot repeatedly beat a group of people at solving an individual problem because each person brings their differing knowledge, unique life experiences and ways of thinking. Which, when pooled together, creates better overall problem-solving abilities.
Combined with the Make the invisible, visible principle, your overall goal should be to allow people to be able to interrogate your problems so you can work on things together.
You can solve a problem together or either individually. Individuals often make great insights in solitude, which they bring back to the group to iterate on their ideas. In contrast, group problem solving allows interactive feedback and live iteration but brings with it many group dynamic considerations that are beyond the scope of this principle (but a good start is here: Be constructive, not destructive)
You've been tasked to create a new domain model for an application. Whilst you are competent with the domain modelling process, someone else is the domain expert. Working on this problem in isolation would be foolish.
Instead, you gather the minds of the domain expert and then at least one other person to validate your domain model.
As your final step, you send the finished domain model to the wider team and business for validation. Using other individuals' "human compute time" to get feedback and solve problems you missed, resulting in a better domain model.
When not to apply
Time is finite; you cannot use more and more "human compute time" of individuals to solve your problems, as they have their own priorities and commitments.