[modeller_usage] successive model building

dear all;
I am going to build a model based on a multiple subunit membrane chanel(a five subunit homopantameric one); it is suggested that I build these kind of models in a "successive" manner, meaning:keeping the template multisubunit and running modeller with the target sequence; then again keeping the template subunits (all) along with the first and second target sequences; then again having template in hand with the dimer made, building the third subunit and so on...
somebody guide me:1- what is the benefit of this successive step modelling?

2- how should I perform this? does anybody have an example file to show me how to add previous models to the current project?

thanks.
adak N.