1) Your responsibilities have increased
2) You have become an integral and important part of the team/project, so it would be costly for them to replace you
3) Your skills have improved
The second and the third point rely on the fact that you'd be probably able to get a better salary elsewhere.
The first point is mostly about fairness. By assuring that you're treated fairly, they can be sure that you do your best.
The second point is the least likeable reason for me, because in that scenario you're not making the judgment from your perspective but from theirs (by realizing your salary potential by not screwing them).
1) Your responsibilities have increased 2) You have become an integral and important part of the team/project, so it would be costly for them to replace you 3) Your skills have improved
The second and the third point rely on the fact that you'd be probably able to get a better salary elsewhere. The first point is mostly about fairness. By assuring that you're treated fairly, they can be sure that you do your best. The second point is the least likeable reason for me, because in that scenario you're not making the judgment from your perspective but from theirs (by realizing your salary potential by not screwing them).