The crucial difference between catch when and catch { if (...) throw; }:

When you use throw; inside a catch block, the stack frame has already been unwound up to that point, altering the original call stack.

Exception Filter:

try
{
    await ExecuteRemoteCallAsync();
}
catch (HttpRequestException ex) when (ex.StatusCode == HttpStatusCode.ServiceUnavailable)
{
    // Handles only 503 without unwinding if exception is 404/500!
}

Exception filters evaluate the condition before the stack unwinds, preserving the pristine crash dump and stack trace in crash reports!

Exception filters also avoid unnecessary stack unwinding performance penalties if the condition evaluates to false.

A great feature in C# that is often underutilized.