In the case of Rome Total War it just didn't fit all one a single CD Rom which have about 700mb capacity so they had to use mutliple discs to fit the game on. The later releases like the Gold Edition with the Expansion pack is on a DVD which can hold 4.7GB.
As for Combo packs, depending on the succes of the title the developers might decide to release a deluxe or gold edition later on which includes the base game and expansions which is convienent for people who don't have the game yet so they can obtain the whole deal in a single purchase and at a nice price too.
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