When you have a lot of issues, it can be hard to get an overview. By adding a weight to each issue, you can get a better idea of how much time, value or complexity a given issue has or costs.
You can set the weight of an issue during its creation, by simply changing the value in the dropdown menu. You can set it to a non-negative integer value from 0, 1, 2, and so on. (The database stores a 4-byte value, so the upper bound is essentially limitless). You can remove weight from an issue as well.
This value appears on the right sidebar of an individual issue, as well as in the issues page next to a distinctive balance scale icon.
As an added bonus, you can see the total sum of all issues on the milestone page.