I figured it had something to do with Death being Hel's portfolio. Like sure, the other gods might have more than 10 million souls, but because Hel is the goddess of death, dead souls are far more valuable to her.

That was mostly due to speculation as to why Thor and Loki might come up with a scheme to minimize the number of souls she would get.

Basically, if there is more death, Hel gets power that the other gods wouldn't, because they are less concerned with death (or go out of the way to prevent it in their followers, since most mortals usually tend to not like their life being shortened.)

The income is useless if you don't know how to use it.