The name of a user value can generally be used as part of a roll expression in place of the value it represents.
For example, if you have a user value named "Wisdom" with a value of '8', the following expression is equivelant to "d20+8":
2d20 + Wisdom
The real advantage of this, is if you have multiple rolls which use the "Wisdom" value, you only need to update the user value, and all roll expressions will use the new value from then on. You can also use the same roll expressions across different characters - simply have a different "Wisdom" value in eahc character's Value Sheet.
For more information on ways to incorporate user values into expression, see the relevant section of the
Dice Notation documentation.