It's amazing how quickly people will attack somewhere else when they have to pay to attack you, and shutting down activated abilities or making tough choices is a real problem for people
Ancient Spring , for instance, taps for blue mana but can be sacrificed for white and black mana
Yawgmoth provides a similar card draw option and also incredibly relevant text for the mirror, having protection from Humans as well as being able to turn creatures that would otherwise die to removal into additional cards while shrinking opposing creatures
This isnt just to sell cards, though that was a huge part of it, but more importantly, it was just because new cards are fun and players are excited to get to use them
I really love that they still use the old "galaxy" holo prints for promos even though the normal holos in sets are different now
Increasing a creature's power after attackers are declared won't cause a training ability to trigger