Providing support for s390x(Mainframe) and ppc64le(PowerPC) would allow the community to build on those architectures.
Many open source communities build there software for a wide range of architectures. Having a one stop shop for a CI/CD platform that can build all the architectures need would be huge. Currently, many open source projects need to split their builds/tests across multiple CI/CD system making it hard to manage.
In addition to the open source community, many big companies rely on s390x and ppc64le architectures for their core systems. i.e Big Banks, Airlines, Credit Cards, etc
If CircleCI add support for these architectures it would allow them to approach new customers and communities.