Mostly it's the resistances the combination provides. Ghost/Steel is the 4th most resistive type combination based on the left side panel of the dual type chart here on the site. Specifically, the typing resists many of its own weaknesses -- Steel is weak to Fighting, but Ghost is immune to Fighting, rendering Steel/Ghost also immune to fighting.
More broadly speaking, Ghost/Steel has no 4x Super Effective weaknesses, and only 4 2x Super Effective weaknesses. That's incredible. Building on that, the combination is immune to 3 types, has a 4x resist against 1 type, and a 2x resist to 8 types. The remaining 3 types are neutral damage. So, a Ghost/Steel type Pokemon will take half damage or less from 12 types, and takes double damage from only 4. Defensively speaking, it's pretty insane.
Looking at the types that are super effective, we have Fire, Ground, Ghost, and Dark. Ghost, like Dragon, is weak to itself -- but this also reduces the need for coverage moves, because having a Ghost STAB will also cover your Ghost weakness. Fire and Dark are both popular types due to their inherent broad coverage, which also means that you probably also have thought about both types already when building your team. Which leaves only Ground, which is extremely easy to deal with -- it is weak to Grass, Water, and Ice, all of which are common types for attacking moves. These facts, taken together, mean that it is extremely easy to slot a Ghost/Steel type into a team, since basic teambuilding will deal with most of the pitfalls in using a Ghost/Steel type Pokemon before even looking at specific Pokemon to use.
Finally, we look at the Pokemon that has Ghost/Steel typing: Aegislash. Yes, it is the only one out of the entire 898 that has this typing. This kind of tells us everything we need to know about how powerful this type combination is. But more than that, Aegislash is itself an extremely addition to any team, through the combination of its ability Stance Change and its signature move King's Shield. The short version is that when you think your opponent is going to hit you with an attacking move, you use King's Shield. It's like Protect, which blocks all damage and effects from affecting Aegislash. Additionally, through Stance Change, the Attack and Sp Attack stats are swapped with the Defense and Sp Defense stats, respectively. Your glass cannon becomes a tank, while also blocking the first hit it takes. Then, after deflecting the nastiness your opponent just tried to throw at you, you use an attacking move, which triggers Stance Change again, swapping Defense and Sp Defense with Attack and Sp Attack, respectively, letting you hit like a freight train.
