This saying is true because when people come together and stand in peace and United toward a driven purpose or goals shared, they will always come out victoriously than where there's division, hatred and disunity.
Even in your business or in firms only one person cannot keep pushing to get to its goal , there need to be ideas and efforts shared from other people working there, the manager, marketing team, producers, everyone needs to work together to keep the firm going.
The statement used to be true, but not anymore. People are not united anymore. Agree on something with a group of people and see how many are willing to take a bribe to betray you. Nowadays, anything that someone wants to do with another person or group of people, they should be prepared to make it legal with a court agreement, no matter how small it is. Including family members. In this world, no one would want to do anything for free.