I think constraints limit the set of possibilities to wander in, if they're good, that will result in more efficent use of effort towards something productive.
As for examples:
  • Legos, built around the idea that all the bricks fit together through one type of connector.
  • The world wide web being built around a simple markup language, allowing it to evolve slowly as its usage increased and speeds grew.
  • This is kind of weird, but, languages with a certain set of symbols representing simple sounds.
  • The base 10 number system with a seemingly random base, we've got ten fingers, that'll help, let's roll with it.
I might have stretched the meaning of what a constraint is, which is quite ironic.