Collagen is made out of mainly 3 amino acids:(hydroxy)proline, lysine, and glycine. So in theory collagen powder is giving you very high levels of those specific amino acids which would help promote collagen formation. I can’t say I’m familiar with the literature and if there is any data behind that - but thats your reason

As mentioned by the other buy, something like whey powder or meat will have a very different amino acid profile compared to collagen. Like fatty acids and carbohydrates, amino acids get lumped together as a group, but they all have very different effects in the body (including competition).

Most people don't need to care, but amino acid balance can sometimes be a significant factor to health issues because of all the interactions involved, especially if intake has been tilted by a particular way of eating. It's reasonable to balance out whey protein with collagen.

Honestly the topic is so convoluted that with my current grasp of if, I can't currently make a useful or broadly applicable statement beyond "some people find collagen or glycine supplementation beneficial for their X". Individual trial and error basically, much like neurotransmitter affecting medication.