The romulan embassy actually has several benefits, once it's fully kitted out.
-Doffs: For each level of one of the embassy categories, you can buy and unlock +1 doff assignment slot. So, if you get all three unlocks you get +3 slots, or a total of 23 potential active doff assignments
-Doffs pt 2: You can also buy some special doffs via the embassy consoles, romulan (and maybe reman?) doffs. For those who like having special doffs.
-Boffs: You can get special romulan boffs which have some nice traits. The big/important one is that they come with a covert or something trait that grants a bonus to your crit chance, and said bonus works on both ground and space. At least one of the tac boffs also comes standard with torp spread 3 in the lt. commander slot, which is a sought after ability.
-Reinforcements: You can buy consumable reinforcement items, which summon romulan/reman troops or ships, who either beam in or warp in and help save your butt. You can get similar things from the fleet starbase, but those are all federation ships. The romulan/remans have different abilities.
-Consumables: Honestly I don't know much about these, other than they're there. There's several consumable items you can pick up from the embassy and use in combat.
-Consoles: The embassy has special sci consoles that have some extra functions. They function as regular sci consoles but also buff plasma damage, and either increase or reduce your threat generation. They're pretty useful.
-Kits: The embassy sells MK X to XII kits. The higher level kits provide an extra ability to your captain, and have an ability at rank IV (instead of capping at rank III like normal kits). For example, there's an engineer kit that adds "quick fix" to the the normal fabrication kit, and gives you turret fabrication IV. The level IV turret shoots enemies with phasers/disrupters at long range, but for close opponents shifts to shooting plasma flame thrower.
All that plus some other minor stuff, and possibly some other things I don't know or forgot about. Most of it you have to buy with fleet credits (and usually dil) once it's unlocked, but it's like all bind on pickup so you need the embassy upgraded to even have the option to pick the stuff up.