There are two reasons to buy this expansion:
One is what it says on the tin: You can play as Byzantium and other Orthodox Christians like Russia, and restore Rome in a special event.
The other, however, is critical to any playthrough of CK2, and that is that you have access to retinues. Retinues seriously change the game's balance of power in favor of large empires, and if you have any designs upon a conquest of Europe, you're going to need their power.
Retinues are theoretically "standing armies", but in play, they are like mercenaries that cost no upkeep except when replacing losses. Although they are less numerous than levies, with proper stack construction, they are capable of beating levies triple their number and fully exploiting the dodgy rock-paper-scissors mechanics of the game.
Basically, once you get to empire-tier realm size, you functionally never have to raise your levies again except to take on another superpower.
Since you do not need to dismiss them before declaring war, you can declare a conquest or holy war on a small duke-tier realm while actually already having your army assembled and halfway marching across the border, already. This allows you a first strike capability that gives your side significant advantages. If you build an archer-heavy retinue, you can also sack castles instead of sieging them, allowing you to win small wars in weeks, rather than years. Win wars without even bothering raising your levies! Let your opponents call up their reinforcements - you'll have already won the war while they get their armies to slowly pool together before they start even trying to march towards you! In fact, with retinues in an empire, I often spend much of my time looking for any random target I can even attack while waiting out treaty timers, rather than worrying about how I will maintain my powerbase.
This is even more exploitable with pagans or other raiders - sacking with a retinue of raiders nets you lots of cash fast (although you pay back a lot of it retraining new troops lost sacking) while a smaller force can come in and conquer the sacked and pillaged lands in a war right after the raiders sweep through.
In total, there's a reason retinues were MASSIVELY nerfed in this game, as retinues completely reverse the trend of making it difficult to keep large empires together, and make large empires extremely simple to expand. Even if you face civil war, having potent retinues are game-changingly powerful. Using retinues makes empires MUCH easier, and in fact, basically TOO easy. That said, once you get hooked on them, you can't ever go back to mere levies again.