6.22Why Flow Reference matters

Without reuse:

POST /customer
  ↓
duplicate Account upsert logic

POST /order
  ↓
duplicate Account upsert logic

POST /case
  ↓
duplicate Account upsert logic

Better:

POST /customer ─┐
POST /order    ─┼→ ensure-account-subflow
POST /case     ─┘

Same reason you use helper functions/classes in normal code.