Currently there's not really any safe way to resize a Vec from the left, and we'd rather not roll our own unsafe for it.
So what are the use-cases for this? well:
The left-pad case is an interesting one, but personally the emulation case is way more useful. And if the allocator could be adjusted to natively support it, that'd mean even more performance where it matters. But even without allocator support, at least the compiler/stdlib would be able to do optimized unsafe code to handle it, while providing a safe API.