Think of BMT as a filter between your Softstep and Live. Each time you send a midi message, you can choose or not to use BMT to transform it for your needs. You can also ask BMT to send messages to the Sofstep itself, typically to control the leds and/or display.
For instance your input can be a note, cc or keystroke and be changed into any other note, cc or keystroke as well, using simple or complex rules, timers, counters etc. Endless possibilities, believe me.
BMT doesn't replace the Softstep Editor but is perfect to avoid using the hosted mode. I personally use the Standalone mode and BMT for all complex messages invoking rules : if… then… that you can program to send different values depending on which/how many press(es) (1st, 2nd, double etc.).
Again, you can search with 'mocker' as the criteria and you'll come across some posts where I describe in detail my main uses of BMT and the Softstep.
OK, one (complex) example :
I lay down a nice chord into a looper, bowing some harmonics on my double bass (through a a 2C Aether plugin long and mysterious reverb and an Eventide H9 harmonizer). I set up the SS's nav pad to pitch the chord a 3rd up for 30s and back to the root by itself, indefinitely. Nav pad value (Y increment as I'm in standalone mode) is set to 0-1 so north triggers a timer that sends the right value to the pitch knob, while south can go back to 0 when I decide and kills the timer. Thanks to BMT.