What is the benefit of moving an antenna to the outside of a house/building?

The antenna offers the opportunity to receive the signal from an outdoor environment which is not prone to the multi-path fading experienced in an indoor environment.

There is a 10-20 dB advantage when receiving the signal in an outdoor open space environment as compared to a cluttered indoors. This together with the antenna gain provides anything between 20-30 dB gain when using an external antenna even if the antenna gain is only 11 dBi. So the received signal will be better than -80dBm.