Sewers have rats, lots of rats.
Most people that run dungeons by themselves use Alvarins for 2 main reasons: the fast running speed, and Alvarin sight.
Alvarin sight is super OP to see other players through walls. Its basically a wall hack radar. There is just nothing like it for sorrounding awareness which is very important when playing solo or small groups.
Running speed and speed boost clades are a no brainer. Once you see the enemy, most often then not, you’ll have to run if they are a big group. There is nothing that lets you survive better than foot speed, especially if you see them coming with Alvarin sight.
Other clades like, Stalker (monsters dont aggro as much), Aerialist (50% less fall damage), Nightvision are also very handy in dungeons. Alvarin are just made to be rats.
While other races are better at straight up fighting (thursars for 1 on 1, Ogmirs tanking lots of damage), nothing comes close to Alvarins as far as tools to rat.