Often as a lazier way to press the face buttons for slow games. But also custom turbo patterns for fast games. E.g.:

• In Genshin Impact, the back buttons are all face buttons, but with select ones set to turbo for automatic item pickup, or dialog skipping, etc.
  • Also very comfy to use them while swimming
• In Hades II:
  • L/R4 are the two shoulder buttons for comfier portal/character interaction
  • L/R5 do autoattack and autospecial on turbo
• In Valheim, one lets me Dodge with a single button press instead of the chord the game demands you use
• In Balatro
  • L/R4 switches hand sorting modes, which has no in-game shortcuts, still
  • L5 restarts a run on long press
  • R5 quits to main menu and resumes with a multi button sequence, to "soft reset"
• In Tabletop Simulator, the most common actions like clicking, selecting, flipping a card, and drawing, are all mapped to the back buttons for ergonomics, freeing face/shoulder buttons for more advanced stuff
• In Minecraft, various back buttons are used to enable different overlaid controls when clicked/pressed based on the modpack.

Etc etc!