Standard/OS threads in Java use about a megabyte of memory per thread, so running 256 threads uses about 256 MB of memory before you've even started allocating things on the heap.
Virtual threads are therefore useful if you're writing something like a proxy server, where you want to allow lots of concurrent connections, and you want to use the familiar thread-per-connection programming model.
Only address space of 1 MB is reserved (which can still be a problem), actual memory usage is limited to the memory pages that are actually accessed by the program within that address space.
