Secateurs why choose an anvil, bypass/parrot-beak or double blade type?

There are varying qualities of secateurs from budget to professional quality.  But why are there different types of blades fitted to secateurs?  Usually, the range extends from bypass (or parrot-beak type), anvil and double blade. Bypass or Parrot-beak secateurs:  as the name suggests the blade bypasses the counter blade to make a cut.  (i.e. the…