Well, I am not an expert on this, so many other more knowledgeable SF members can correct me if I am way off.
I don't know how essential breaking in on carpet is, but many very experienced and expert members on this forum believe it is the best route to go, and I see no reason to doubt them.
Breaking them in while indoors on carpet and also for a limited amount of time I believe allows the shoe leather sole to start to flex and loosen a little (allowing it to bend). The leather sole breaking in and starting to flex allows you to walk without digging your toe into the ground. If you break them in on concrete then you may start to get irregular or excessive grinding on the toe of your shoe/boot. Also, if you wear them inside on carpet, in some cases if the seller allows returns, then you are able to not scuff the soles, and if you also do not bend enough to create creases in the leather or rolls in cordovan, then you may be able to return them if you find they don't fit.