The main reason is to make easier team organization thanks to marks, the main purpose is for trainings like while giving trining for Fractals CMs encouter scenarios, making easier to understand/explain with a visual reference for new players or while practicing strats with a statics on those encounters.
the main problem I can think is limiting the quantity of people on the instance but the way I see is like was done with raids (you can't enter on raids instances without making an raids squads), the point is if you are in a party/alone you should be able to enter on the intance, if you are tagged up (commander tag) you need to activate the fractals mode (or just enter alone like is righ now) poppoing tag on the instance shoulde be activated automaticlly on fractals mode.
Yes this is an optional feature and people can still doing trainings/practices/runs like before, but the amount of trainigs are limited by -1. People who aport to get an excelent community (or at least good) on high-end contents -2. Mediuns to explain or make undestand difference between normal modes and CMs -3. Time that can be gived from trainers -4. Good organization while doing trainings or just leading people.
(what I mean here is: there is a lot of peep who want to just get loot on scenaries like T4 Fractals/CMs but they still don't know who to clear mechanics correctly or just how to even clear the whole fractal/content. So is good get loot yes, it is, but one thing is get fun orther thing is get loot while making an effort to get it and the final thing is people who just want to be carried to get "free loot/legendaries/achivements")
I really love Fractals content and I've builded my principals toons for it, but there is a lot of people who still having basic doubs like: CC skills, Mechanics, encounters in general, etc... Not only on CMs those doubts are on pve in general, so in my oppinion tags (commander/mentor) are not only for situations like "oh, I want to do this event but need peep... imma tag up" an commander/mentor should do like the words say "guide"/"lead", taking into account both cases there is a lot of people who want to learn the game but we still lacking on tutorial (in-game) or just some game mode like Fractals CMs what have an "barrier" becouse they are limited just to people with tons of KP.
The main point of all this poll is give the posibility and the facilities to the community to help other players who want to learn and spend some amount of time on those "high-end" contents or just getting better and of course still having the posibilities to get fun with all the community/friends.