With the cloud, in a lot of cases you can have additional regions that incur very little cost as they scale dynamically with traffic. It’s hard to do that with on-prem. Also many AWS services come cross-AZ (AZ is a data center), so their arch is more robust than a single Colo server even if you’re in a single region.
Cross region from on-prem to the cloud for a website is easy. In fact, as long as you don't buy into "cloud native" ("cloud lock-in"?), it's probably more cost effective than two on-prem regions or two cloud regions.
Being able to choose from so many different Availability Zones in so many different regions is one of the best things about AWS. Combined with sophisticated routing strategies that Route 53 supports allows for some very effective designs.
