Common practice usually accounts for a decoupled system or independent response of the structure from its underlying soil (and vice versa). While this approach is reasonable for light structures founded on relatively stiff ground, it can lead to an inaccurate dynamic response of a heavy structural system resting on a relatively soft soil (Figure 1). Soil-structure interaction depends on the details of the problem at hand and must be evaluated on a case-by-case basis.

(a)                                                      (b)
Figure 1. Response of the two structures under Takatori (Kobe 1995) Earthquake
(a) While soil structure interaction can be neglected for some structures
(b) It completely influences the response of other structures
[Anastasopoulos et al. (2010)]

Some of the important facts of why SSI is important can be listed as follow:

The natural period of the structure gets longer as the stiffness of the soil gets smaller. Ohba (1992) proposed a correlation between height of a structure and stiffness of the soil with the natural period of the structure.

Radiation damping will generally cause the total damping of a soil-structure system to be greater than that of the structure itself.

Soil – structure interaction may cause basement motions to differ from those of the free field.

Load distribution in members may be different following an analysis that includes soil-structure interaction and could cause differential settlements and cracks propagation.

References:

Anastasopoulos,  I.,  Gazetas,  I., Loli, M., Apostolou, M., and  Gerolymos, N.  (2010). “ Soil failure can be used for seismic protection of structures” , BULLETIN OF EARTHQUAKE ENGINEERING, Volume 8, Number 2 (2010), 309-326, DOI: 10.1007/s10518-009-9145-2