What makes it better than Vagrant for this use-case?
Doesn't Vagrant spin up full VMs? Incus/LXD/LXC is about system containers. So like Docker, but with a full distro including init system running inside the container. They are much faster to spin up and have the best resource sharing possible.
Vagrant has a provider plugin for lxc. Please check my reply to the parent commenter for more explanation.
Vagrant is not the right comparison against Incus for this use case. Vagrant is used to spin up VM or system container instances that are configured for software development and testing. But vagrant doesn't create those VMs or containers by itself. Instead, it depends on virtualization/container providers like VMware, Virtualbox, libvirt or lxc. In fact, you could create a provider plugin for vagrant to use Incus as its container/VM back end. (I couldn't find any, though.)