What is the advantage of MISO technology in communications?

Multiple input single output refers to the scenario where you have a transmitter with multiple antennas and a receiver with a single antenna. That is, the channel sees multiple inputs from the multiple transmit antennas and a single output at the single receiver antenna. With that being said there are many benefits of multiple antennas:

1. Wireless channels undergo what is called fading and can sometimes experience a so called "deep fade" in which the channel strength drops low enough to a point where reliable communication is no longer possible. Say we have 10 antennas at the transmitter and a single antenna at the receiver, then there are 10 wireless channels, one for each transmit antenna. Intuitively you can think the probability of a deep fade event should now be less since all 10 channels would have to experience a deep fade at the same time. This is referred to as transmit diversity.
2. With multiple antenna elements we can perform what is called beamforming. Beamforming allows us to amplify signals we send in certain directions and can even further attenuate signals in different directions (pointing a null beam). Many beamforming techniques have been developed including adaptive algorithms which perform beam pointing "on-the-fly" to try to maximize SINR for example.
3. Multiple transmit antennas are necessary for certain space-time block codes (STBC) which boil down to transmitting specially modified versions of the same signal during the different time slots of transmission. A common and simple STBC is the Alamouti scheme.

These are just the first few that came to my mind but, multiple antenna systems are an active research area and with 5G pushing massive MIMO there are many groups doing work in this area.