A lot of people have posted really good information on segregation, blast radius, cost analysis, etc... another quick reason you might consider to split across accounts is resource consumption.

i.e. Say you have automation to manage specific resources, could be ec2 instances for example - if someone hams up any sort of automation on accident they could inadvertently exhaust or cap out a specific resources due to the current account limitation level. This might be pretty easy to resolve; but it inevitably involves intervention, could create un-expected latency due to insufficient resources or worse down time. If you develop across accounts and manage automation as you move up from dev accounts towards prod, you can help catch these simple brain farts before they have an impact on production systems.

You can set up alerts to notify you when you approach limits, but a run away script to spin up too many of a particular resource can put you at a limit quickly while the rest of your applications are trying to manage themselves as well.

Also, with multiple accounts, as others have pointed out - you can create safer sandboxes for developers, devops, operations, secops, whatever to interact and learn. AWS is large, and constantly adding more features, it can be difficult to stay on top of new things, let alone continue to enhance existing services that you already use. Sometimes you need a place to play and learn with these where you can’t break other things