On Sunday night's episode of Game of Thrones, the Night King did what many fans hoped was impossible; he killed a dragon. So what does the Night King want with Viserion on Game of Thrones? It seems he wanted a dragon in his army for a very specific reason.
When Jon led a group beyond the wall to capture a wight, most fans expected the trip wouldn't end well, but most thought it would be a human to die, not a dragon. However, when Gendry ran to the Wall to send a raven and inform Daenerys that the rest of the group was trapped by wights, Dany wasted no time and grabbed her dragons to head north and save the day. Though Drogon and Viserion successfully killed a number of wights and rescued the majority of Jon and his crew, the Night King still managed to kill Viserion with his ice spear.
After killing Viserion, the Night King had wights use chains (which they got from... somewhere) to drag Viserion's body out from the icy waters he fell into. He then used his power to awaken Viserion, turning him into a wight dragon, or an ice dragon, if you will. It's unclear if Viserion will still breathe fire or if he'll now breathe ice. Either way, having a dragon on his team is definitely helpful for the Night King.
Specifically, the Night King probably wants a dragon to aid him in bringing down the Wall that protects Westeros. The Wall has stood for thousands of years, but as Jon has pointed out, it hasn't been refortified in ages. Still, the Wall is probably still very strong and it would definitely make the Night King's life much easier if he could just tear it down rather than having to try and walk his whole army through the gate.
Whether Viserion still breathes fire or not, he could definitely take the structure down, I would think. And if he can't do it for some reason, he can also fly over it, bringing the Night King and some of the White Walkers and wights along for the ride. No matter how the Night King decides to use Viserion, having a dragon on his side will definitely aid his cause and will help him stand against Daenerys' other two dragons.
Though it'll be sad to see Viserion face off against his brothers and mother in the future, it will also be a pretty epic battle. Who would win in a face-off between a dragon and resurrected dragon? Fans will probably find out soon enough.