MIMO helps in achieving array gain, spatial diversity gain, spatial multiplexing gain and interference reduction. These gains are described in brief as follows,

Array gain:

Array gain is the increase in received SNR that results from a coherent combining effect of the wireless signals at a receiver. The coherent combining is realized through spatial processing at the receive antenna array and/or spatial pre-processing at the transmit antenna array. Array gain improves resistance to noise, thereby improving the coverage and the range of a wireless network.

Spatial diversity gain:

As mentioned earlier, in a wireless system the signal amplitude fluctuates or fades. Spatial diversity mitigates the effect of fading by transmitting the same signal through multiple antennas via different paths. The receiver receives multiple copies of the transmitted signal in space, frequency or time. With an increasing number of independent copies (the number of copies/paths is referred to as the diversity order), the probability that at least one of the copies does not experience a deep fade increases, thereby improving the quality and reliability of reception. A MIMO channel with $N_{T}$ transmit antennas and $N_{R}$ receive antennas offers $N_T*N_R$ independent fading links, and hence a spatial diversity order of $N_T*N_R$.

Spatial multiplexing gain: MIMO systems offer a linear increase in data rate through spatial multiplexing, i.e., transmitting multiple, independent data streams within the bandwidth of operation via different path. Under suitable channel conditions, the receiver can separate the data streams. Furthermore, each data stream experiences the same channel quality as experienced by a single-input single-output system, however enhances the capacity by a multiplicative factor equal to the number of streams. In general, the number of data streams that can be reliably supported by a MIMO channel equals the minimum of the number of transmit antennas and the number of receive antennas, i.e., min$(N_T, N_R)$. The spatial multiplexing gain increases the capacity of a wireless network.

Interference reduction and avoidance:

Interference in wireless networks results from multiple users sharing time and frequency resources. Interference may be mitigated in MIMO systems by exploiting the spatial dimension to increase the separation between users. For example, in the presence of interference, array gain increases the tolerance to noise as well as the interference power, hence improving the signal-to-noise-plus-interference ratio (SINR). Additionally, the spatial dimension may be leveraged for the purposes of interference avoidance, i.e., directing signal energy towards the intended user and minimizing interference to other users. Interference reduction and avoidance improve the coverage and range of a wireless network.

It In general, it may not be possible to exploit simultaneously all the benefits described above due to conflicting demands on the spatial degrees of freedom. However, using some combination of the benefits across a wireless network will result in improved capacity, coverage and reliability.