Wood panels set in frames tend to shrink over time, exposing an unfinished stripe at one or both sides. Even paint won’t keep this from happening, as demonstrated in the accompanying picture.
The only effective way to keep panels from showing the stripe is to stain or paint them before assembly. Gluing them in won’t solve the problem because this will cause the panels to split, which is worse.
With the panel already colored, it can shrink as much as it wants without leaving unsightly stripes.