They're probably using AWS accounts for all or some of the following:

1. Compartmentalization (security incidents and manual mistakes are isolated per account). On the extreme side even some sensitive [micro]services may run in separate AWS a/c.

2. One AWS a/c per environment (i.e. dev, staging, prod).

3. One AWS a/c per large enterprise tenant (in case of the multi-tenancy).

4. Every team/division inside the organization have their own sets of AWS accounts usually with separate billing.

Yep.

Once you get the processes and pipelines built to manage account lifecycle, they provide a lot of valuable compartmentalization properties.