Episode 6. What was the reason of Reva’s existence on this show?

Exactly.

I agree that Reva was not integrated particularly well into the series and the conclusion of her character arc shouldn't have been inserted into Obi-Wan and Vader's duel.

In 'Star Wars Episode II: Attack of the Clones', Tusken Raiders kidnap Anakin Skywalker's mother, Shmi Skywalker. Anakin eventually finds her, but she is mortally injured and dies in his arms. Seized by a violent rage, Anakin slaughters the entire tribe including the women and children. In 'Star Wars Episode III: Revenge of the Sith', he totally embraces the dark side to save Padme, slaughtering the Jedi children.

Reva survives the youngling massacre, becomes an Inquisator and committed all kinds of atrocities for the Empire in order to get close enough to Anakin to extract revenge. She realises that she can't overpower Vader, but gets the chance to kill Luke and inflict the ultimate revenge, the death of Vader's son. However, unlike Anakin, this is the line she won't cross.

As opposed to Anakin, who becomes an unrecognizable monster, obsessed with vengeance against Kenobi, Reva shows that it is possible to turn away from the dark side and that the cycle of violence and retribution could be broken.