Pride Comes Before a Brawl - S1-E7
Continuity mistake: When Hercules and Nemesis are in the boat discussing her assignment, the shoreline behind Hercules doesn't move behind him as they speak (so the boat is lying still). But whenever the camera cuts to Nemesis, you see the background slowly moving away (so the boat is moving).
Other mistake: Callisto's sword is missing. She's the Warrior Queen, so she should have her sword. When Callisto appears in the Xena Season Two episode "A Necessary Evil", her sword has reappeared when Xena frees her from the Labyrinth of the Gods for her help with defeating Velasca.
Armageddon Now: Part 1 - S4-E13
Character mistake: Callisto's sword is missing. When she appears in the Xena Season Three finale "Sacrifice", her sword has reappeared. It is never explained why Callisto doesn't have her sword during her three appearances on Hercules the Legendary Journeys.
Answer: Most shows do flashbacks to show how things begin or what circumstances brought them to their current predicament. The modern episodes was to show how their legacies live on. Hercules changing his name to Kevin Sorbo and starring in a series about himself or Xena, her soul being reborn to continue her fight against evil.