#S_(N)1# reactions are important because, as far as we know, they describe a mechanism of organic reactivity, of chemical reactivity. And they describe a BOND-BREAKING PROCESS, as compared to #S_(N)2# reactions, which are bond-making processes with respect to the rate determining step.
#S_(N)1# reactions are characterized by the loss of stereochemical activity. That is for #CR_1R_2R_3X#, substitution results in the loss of chirality, due to a presumed #""^+CR_1R_2R_3# intermediate.