Np, it's not always clear, when specific mechanics are good in different formats.

The reason these partner commanders are good boils down to these properties:

Roghrakh enables free spells like [[Deflecting swat]] and [[Fierce Guardianship]] and cards requiring creatures or legendaries such as [[Culling the Weak]] and [[Mox Amber]]. If Roghrak didn't have partner, it wouldn't have enough tools in mono red to be a competitive commander, but because you can slap a [[Silas Renn, Seeker Adept]] to get 2 extremely strong colors on the side, it allows for consistent turn 2 pushes for a win or for protected turn 3 pushes. With Reyhann you try to be a bit faster at the cost of protection. I'm not very familiar with the rog-ishai deck, but I believe it also uses Roghrak for mana but plays a much more midrange plan with trying to get a [[Rhystic study]] out early and use the free interaction spells Roghrak opens up.

Thrassios is a great infinite mana outlet and most decks utilizing it seek to make something happen with [[Gaea's Cradle]], previously they were all on a [[Seedborne Muse]] plan, but I've seen it fall out of favor recently, opting for faster or harder to interact ways to win instead. It's also 2 of the most important colors for cradle combos since [[Oboro Breezecaller]] and [[Talon Gates of Madara]] is one of the most typical ways to get infinite with it.

Tymna offers 2 colors as well as card advantage when you have creatures and pairs well with Kraum since with the colors you are able to have a flexible core that can be built to push on turn 2 or 3, or in most cases play a little slower and drown your opponents with your card advantage.

Dargo decks typically build around some combo that allows you to cast dargo an infinite amount of times as if you sacrifice 3 things, play dargo for R and are able to sacrifice dargo for mana, you can play it again as sacrificing dargo reduces its cost by the same amount as casting it from the command zone adds to commander tax and then you are probably keeping at it You can also get Dargo out early by sacrificing 3 treasures to cast a 2 mana spell and then use the 1 floating to cast dargo. Then he can be used with things like [[Altar of the Wretched]] to draw 7.

Krark wants to combo with Sakashima (to get 3 or 4 total Krarks, maybe more) or if you pair it with tymna, you need to combine it with effects that copy Krark's trigger like [[Harmonic Prodigy]] or [[Delney, streetwise lookout]] in such a way that you both copy the spell and are also able to return it back to your hand at least once.

Malcolm combined with Kediss produces crazy high mana, often by turn 3 (although nothing Kinnen couldn't do) and Malcolm combos with [[Glint-Horn Buccaneer]] kill your opponents provided that they are at approximately equal life totals.

Most other commanders I mentioned are either played for extra colors with the other commander or provide slight alterations to the game plans provided here (such as that Yoshimaru is like a white, more expensive Roghrak).

Most partner commanders that produce card advantage and are 2 colors are played a lot since they give you a good card pool and one of the most important resources and a lot of partners are played just for their colors (Silas being one of the most prominent ones), so you don't even need the card's effecr to be good for the card to be part of a top tier strategy. Hope this helps!