In Harry Potter and the Deathly Hallows Part 1 (2010), Ron got injured after Apparating, so Hermione treated him with a spell.
Yet afterwards he is still shown wearing a bandage: why so?
The wound was closed using Essence of Dittany but it's not a complete solution...so there is still damage to the arm.
Harry wrenched the stopper off the little bottle, Hermione took it and poured three drops of the potion onto the bleeding wound. Greenish smoke billowed upward and when it had cleared, Harry saw that the bleeding had stopped. The wound now looked several days old; new skin stretched over what had just been open flesh.
“It’s all I feel safe doing,” said Hermione shakily. “There are spells that would put him completely right, but I daren’t try in case I do them wrong and cause more damage. . . . He’s lost so much blood already. . . .”
Harry Potter and the Deathly Hallows - J.K. Rowling
So although the immediate problem has been solved his arm is still not completely healed so a sling (not a bandage) is required