Kanna is all about the synergy between her passive and signature card - which facilitates the creation (and with proper deckbuilding, the maximization) of a wide board state. While her passive allows you to heavily pressure a lane, it's also a huge red flag for your opponent to defend it. Cards that scale with a wide board greatly helps tip this in your favor. Kanna's relative durability for a blue hero increases your chances of playing the powerful blue spells.
Kanna has an amazing health pool - the highest among blue heroes by a large margin (next being 8, Luna and J'muy), and pretty high compared to all other heroes. Her tankiness helps cast blue's amazing spells - and in a vacuum that reason alone makes her a viable addition in decks.
Kanna's ability greatly widens the lane she's in. Maximizing this ability should be the main reason to field Kanna in your decks.
Keep in mind it also weakens your other lanes. Creeps won't ever spawn on them! It only manipulates WHERE creeps spawn, not add new ones - it doesn't create NEW power.
This is more nuanced than it seems. Since a creep spawning into a "lost" lane is essentially useless, pulling creeps into a winning or contested lane (which i HOPE is the status of your Kanna lane...) essentially ENSURES an increased value of that creep. If you couple this with cards that improve a wider board (IE Mists of Avernus), you are gaining massive value out of each creep, rather than just having these creeps as an afterthought. So while I mentioned in the above blurb that there is no net gain in power, a deck properly built to maximize Bringer of Conquest can skew this equation to actually yield a large INCREASE in power.
Its inherent synergy with her signature card, Prey on the Weak, can very quickly create overwhelming board states out of nowhere!
First of all, let me make it clear - Prey on the Weak counts ALL damaged units in the lane. Meaning YOUR + OPPONENTS units! This synergizes extremely well with her passive, as the more units you have in lane, the better this effect is. Furthermore, your opponent has to respond to Kanna with units and heroes of their own - adding to the PILE of targets for Prey on the Weak!
Since it only works on damaged units, you're bound to get some effect from this no matter what, as units are constantly bashing into each other. But why stop there? We can ENSURE more damaged exist at any given time.
Improvements like Conflagration and Ignite not only clear the way for your dogs and creeps to rumble into their tower. They also damage ALL enemy units every turn, which means more dogs! You can even damage your own units using spells like Diabolic Revelation. It's no coincidence Kanna's face is on the card.
Cards that scale from number of units (IE Mists of Avernus), will not only scale her passive - it scales this too! Buffing the large mass of doggos from Prey on the Weak is so OBVIOUS that I almost feel bad I have to type it out.
The comparison to Dimensional Portal is quite obvious. I'd say you want at least 5 dogs for it to go even with D Portal. Speaking of which, Dimensional Portal and other spells which generate multiple creeps are also cards that help push the Kanna gameplan a little faster.
While her passive is great, make no mistake, it's ultimate purpose is to power out HUGE Prey on the Weaks. This culmination of little synergies is what you're after when you sleeve up Kanna.
While Kanna works fine in any deck running blue, it's highly recommended you TRY to gain some value off your wider board state. Even a few support cards can allow Kanna to viciously swing the game to your favor in one or two turns.