If you are making sandboxes, you need to put the files in place each time. With ZFS clones, you can keep referencing the same files repeatedly, so the amount of changes to memory needed to create an environment are minimized. Let’s say the sandbox is 1GB and each clone operation does less than 1MB of memory writes. Then you have a >1000x reduction in writing needed to make the environment.
Furthermore, ZFS ARC should treat each read operation of the same files as reading the same thing, while a sandbox made the traditional way would treat the files as unique, since they would be full copies of each other rather than references. ZFS on the other hand should only need to keep a single copy of the files cached for all environments. This reduces memory requirements dramatically. Unfortunately, the driver has double caching on mmap()’ed reads, but the duplication will only be on the actual files accessed and the copies will be from memory rather than disk. A modified driver (e.g. OSv style) would be able to eliminate the double caching for mmap’ed reads, but that is a future enhancement.
In any case, ZFS clones should have clear advantages over the more obvious way of extracting a tarball every time you need to make a new sandbox for a Python execution environment.