They are mercenary units using the dark age of Technology. Very cool fluff, and the models are great. They are modeled very well, considering they were made like 20-30 years ago initially and have retained some of the classic shape, whilst evolving considerably.
I am referring to the shape of the upper torso hull which is quite antique now.
Ancient model

40k Final Liberation game from 20 years back

Why wouldnt they be in games? They are for Imperials, and 6th edition is still relatively new looking to transition into flyers and mega units being released left right and center.
They are mercenaries so we may even see Orks taking them (because Orks, Chaos *Dark Adeptus* can take Imperial Guard Allies... remember Blood Axes do deal with umies)
There will have to be an official rules set for them soon as they are released officially shortly.
Speaking from a purely aesthetic/ model perspective - the faces for model options given are relevant to many different races out there. The good, the bad and the ugly. You could use those models for Chaos, Marine(BA,DA,SW,VANILLA, GK, SISTERS), Ork or covert the hell out of them. That means every army now has a way to even the playing field vs Tau/Eldar.
Remember they are not titans! The Imperial Knights are a similar Class to Riptides and Wraith knights which are already used in regular games, and
Tiptides are already tipping the scales of games due to their sheer size, and even messing up tournaments a bit.
Images shown to help visualize why its important imperials get them.
With
Rip tides being used with t6, 5 wounds, 2+ save, 5+ invul and a 4+ invul charge. around
300-400 points
Eldar get the Wraith Knight.. t8, 6 wounds, around
250-350 points > can be used in standard game of 40k
.JPG)
The Imperial Knights are like armour 13, 4+ invul for a single turn,
350-390 points and need to pretty much be destroyed 3 times in a row to go down.

Taking in terms their size, they would put Imperial Guard, Marine and Chaos on par with their Eldar/Tau - Wraithknight/Riptide counter parts.
In a Nutshell it is fair marine and Chaos get this guyDont get me wrong, its possible they dont, I am not officially saying they do as the rules arent out yet, but saying
they have every right too and it would be justified giving comparable points costs and unit damage typology.
For one plastic box model, this
could be a huge door opener in many ways.
aka Alexander / Eye0fHoruS.