The standard build uses a "least common denominator" Intel instruction set (SSE4), but the odds are extremely high that the machine on which you're running tensorflow supports 4.2. Building from source allows you to use the most up-to-date instruction set (the default configuration script at https://www.tensorflow.org will do it automatically).

I've seen dramatic (>50%) reductions in processing time on test scripts by building from source. Note that those tests were built primarily for my own education, not for benchmarking. But the speedup was so dramatic that I couldn't help but notice and probe a little deeper. YMMV depending on the particular application, whether you are using GPU computing (I am not), etc.