The main benefit of arm lift is removing the loose, hanging skin that develops on the upper arm. While diet and exercise are good tools for burning fat, they are unfortunately unable to reduce skin that has already stretched and is no longer taut enough to retract. The only practical solution for addressing excess skin is plastic surgery.
As for fat, the upper arms are an area of the body where fat clings most stubbornly, especially in women. You may notice that weight loss improves contours in other areas before your arms.