Installing 20 or more weapon mods also grants the player a trophy or achievement, which may be a big draw for some of you. It also gives players the capability to specialize a weapon more towards their own style of play. They can also improve weapons to such a degree that they rival the unique weapons in power and usefulness.
The answer to the question is a fairly simple - it allows weapons to be made much more powerful than their base statistics, giving them more stopping power.
