Our standard Buffer (not Buffer+) is just a single input buffer. There is no buffer at the loop in. The loop in and amp out jacks are simply there for convenience of pedal board routing, and set up and tear down. It basically makes the buffer the central routing point of your pedal board.
In the buffer+, we have a buffer at the loop input because it's essential to the extra functions of the buffer+, such as the noise filter and output boost. Alternatively, you can also use the buffer+ as a buffer for your effects in front of the amp, and as an fx loop buffer by sending the fx send from your amp to the loop in.
In other words:
Our buffer has a buffer on the guitar input and tuner out
Our buffer+ has a buffer on the guitar input, tuner out, and amp out
Here is a more accurate diagram of the buffer+ signal chain: