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.