Virtualization is being adopted in many different environments and situations. If you are a developer, virtualization can provide you with a contained environment where you can safely do almost any sort of development safe from messing up your main working environment. If you are a systems administrator, you can use virtualization to more easily separate your services and move them around based on demand.
The default virtualization technology supported in Ubuntu is KVM. For Intel and AMD hardware KVM requires virtualization extensions. But KVM is also available for IBM Z and LinuxONE, IBM POWER as well as for ARM64.
Qemu is part of the KVM experience being the userspace backend for it, but it also can be used for hardware without virtualization extensions by using its TCG mode.