Making DoSomething a template because span is a template is a non-sequitur.

If DoSomething works with untyped bytes, it should require a std::span<byte> (or const byte if read only). Incidentally the standard provides a convenient as_bytes(std::span<T>)->std::span<byte>; There isn't an as convenient helper to convert a singular object to a span of bytes, but it is easy to write.

As to why one should use span, is that a) it helps making sure that the size travels together with the pointer for some additional safety, b) it is more convenient to work with byte ranges than void ptrs (which do not support pointer math), c) helps a bit communicating intent: in C++ void* are used more often for type erasure than for byte related things.