One reason for collaboration is to raise the bus factor.
For small to medium-sized applications, it's not hard to get a single good developer to crank out feature after feature... but they're the only one that understands any of it. Then that single developer is hit by the proverbial bus (or Corona, or retires, or resigns, or whatever...) and you have important software without any maintainer.
That might be OK for some startups where the expectation is that the code will be thrown away and replaced by something better pretty quickly, but for slower organizations, that's often a nightmare scenario.
For small to medium-sized applications, it's not hard to get a single good developer to crank out feature after feature... but they're the only one that understands any of it. Then that single developer is hit by the proverbial bus (or Corona, or retires, or resigns, or whatever...) and you have important software without any maintainer.
That might be OK for some startups where the expectation is that the code will be thrown away and replaced by something better pretty quickly, but for slower organizations, that's often a nightmare scenario.