"We have met" = We don't know how many times or where or when.
"We met yesterday" = We don't know how many times or when, but we do know where.
"We met at noon" = We know it's only one instance, we know the time but we don't know the location. We don't know if it is the only time the two people have met.
"When we met yesterday, I realised we had met before" = Time relation is important.
Student or Learner