In control engineering, a single-input and single-output (SISO) system is a simple single-variable control system with one input and one output.
Usually, it is also easier to make an order of magnitude or trending predictions "on the fly" or "back of the envelope".
MIMO systems have too many interactions for most of us to trace through them quickly, thoroughly, and effectively in our heads.
Bode plot, Nyquist stability criterion, Nichols plot, and root locus are the usual tools for SISO system analysis.
Often SISO controllers will be PI, PID, or lead-lag.