Reserve sets the capacity, not the size. If you were to reserve and overwrite the buffer, the size is still wrong. Now if you resize, STL overwrites your contents with value initialized chars.

Also I don't think the Standard requires that data() returns a valid pointer after only calling reserve(), so it's technically UB.

i think you may have misunderstood the standard - data() will always return a valid pointer, but the pointer returned may be invalidated by any number of functions:

    string s;
    s.reserve(50);
    char * p = s.data();   // ok so far
    s.reserve(100);        // now p may be invalid

Of course, you need to call push_back() or insert() to add the data. Actually, insert() wouldn't even require reserve() in the first place.

I guess the idea of uninitialized resize is that you reserve the space, bump the size, and then you can directly set the new members with operator[]. If your input data is not contiguous, and would need to use push_back() otherwise, you can save a few if-checks. But that's pretty much it, I assume.