A topper changes surface feel; it does not rebuild the bed.
Added height may require different sheets.
Use a topper for a specific surface-feel problem.
What toppers do well
A topper can soften a surface, add a cushioned layer, change texture, or make an older bed feel more pleasant for a while. It can also help two people fine-tune one side of a bed if the topper size and setup allow it.
What toppers do not always fix
A topper may not fix a bed that feels unstable, poorly supported by the frame, or too warm because of the room and blankets. It also adds height, which can make sheets fit differently and change pillow feel.
Thickness and material feel
Thicker is not automatically better. A thick topper can feel luxurious at first but may make movement harder or increase warmth. A thinner topper may be enough if the goal is surface smoothing rather than a major softness change.
Fit and shifting
Toppers can slide, bunch, or change how sheets grip the bed. Deep-pocket sheets, corner straps, a grippy protector, or a better-fitting cover may help. If the topper moves constantly, the comfort gain may not be worth the nightly adjustment.