Catherine's Merciless Pursuit ability is one that change the outcome of a match. Both Skaarf and Adagio have Ults. that can take large chunks of health out of an entire team. But, is you haven't noticed, both have very obvious charge ups before melting (literally in Skaarf's case) 2 or 3 people. If Catherine gets in there and stuns them before their Ults. take action, it can change who gets the Kraken, and ultimately wins the game.

The reason why Journey Boots are so important for Catherine is because I noticed, that as an Initiator/Tank, chasing is a very key part of her role. It may seem odd, but journey boots also have a shield/health increase as well as movement speed and the sprint activate.

Catherine is a very versatile character and plays a key role in team fights. I generally play Catherine when another player is playing Adagio for support. In this case you are still Ironguarding, but Catherine is nearly invincible with an Adagio by her side. It's better to play in a party so you know that the other person won't abandon you and go CP Lane Adagio.

Another good partner for Catherine is either Taka or Koshka. Koshka is even better because of her Ult.'s stun, in which case you will have a double stun. A Catherine-Assassin combo is the perfect gank team, starting off with the stun and having Taka melt somebody is the most amazing feeling ever.
I understand that you don't always pick your teammates. I'm looking at you Krul insta-lockers. This is why it is always a good idea to be able to play more than character.

One more very important detail. DO NOT GO IN ALONE. Diving is a horrible mistake, it is not worth a death for 200 or less gold. You're better off waiting for a good team fight for a chance to ace. Catherine doesn't do much damage on her own. She is meant to make sure that team fights don't end up with a late-game ace that costs you the game.

Catherine can sustain a fair amount of damage, but you should avoid 2v1s at all costs. Catherine has a wide variety of escape methods, including journey boots and her merciless pursuit, but don't get over confident. Glaive can easily stun you towards an awaiting assassin the bushes. Your goal is to level up as fast as possible by stealing jungle farm and stunning enemies with half or less health so others can finish them off, but once you have full build, you shouldn't need someone to kill them for you.