There are several possible reasons to run a stand-alone boost controller. One is that you may be able to get the turbo to spool up faster; mechanical boost controllers boast spool-up speeds that usually exceed even the best ECU-controlled systems. Another is that you might want to have an easy way to dial the boost up or down, without having to dig into the ECU boost control maps. (The TurboXS Dual-Stage Boost Controller is particularly good at this.)
In any case, we stock a full line of both mechanical and electronic boost controls. All the boost controllers we sell are compatible with any turbocharged Subaru.