Decentralized storage taps into latent/underutilized capacity from datacenters and individuals across the world. For example, if you have 10TBs of free capacity that wouldn't otherwise be used, and running a node is pure margin. Most private datacenters are at about 40% capacity.
Another example –> the total cost for me is more than 10x cheaper than AWS - its 7$ for TB/Bandwidth vs $90 on AWS. I can also get 1.8gbps - in my region, which is even faster than AWS, and it doesn't rely on ACL databases (it uses client-side hmacs for file authorization). So, for my use case (static content storage and low-volume CDN usage) decentralized cloud wins. (it's 10x cheaper, faster, and more secure for my needs).