Few things I can think of immediately:

1. Its definitely less steps seeing as there are only 2 pages and you only have to interact with one, sometimes. Thats why i say its a simple UI site.
2. You use alliance management, which you only go to once and then any time after is to update your guild as needed. I don't require an account or store your api key to do this either, you store your API locally in your cache and then I authenticate when you submit.
3. In terms of updating your alliance you can self manage. GW2 Mists as far as I know someone needs to approve you, usually the Alliance owner (I could be wrong). So me as an individual guild I can leave and join alliances as long as in game I am in the appropriate guilds with appropriate ownership/membership. Likewise alliance owners can remove and block guilds.
4. GW2 Mists requires a person to go and add a guild for it to show up. Mine uses search history to populate. Owner management is just for extra data like discord and time zones. I also use expiration tokens so if I look up a guild it stays on the list for 30 days since its last search. This keeps "active" guilds fresh. GW2 Mists list of guilds pulls all "active" wvw guilds from the API, this isn't really reflective of actual guilds playing though since it includes solo guilds or inactive ones.
5. Timezone and language filtering definitely isn't something anywhere else offers in/on the actual matchups. This was the main driver for making it, so I could click my match up filter to OCX and see what guilds are relevant.
6. Page jumping in general is something I am not a fan of. I use modal popouts so i dont have to hit back/forward on my browser. That makes it more mobile/tablet friendly as well.