I mainly have three reasons to do so, especially with the Eldrazi Titans.

1: It's fun. I love homebrewing creatures and races and for the longest time I've wanted to run a campaign with one or more of the titans as (an) end boss(es) or as the result of the PC's failing/completing their quest/mission/campaign/etc.

2: The inspiration is there, why not at least give it a shot? Sure it might come to be/accomplish nothing, but there is a certain satisfaction in knowing "I did this, I've created something that someone may use at some point in time. It might be long after I've withered away and become dust but there's always the chance."

3: It's good exercise. I find a certain feeling of accomplishment when I can look at a piece of fluff and come up with a mechanic to bring up to it. For example: the Eldrazi subtype, The Promised End and The Aeon's Torn abilities, a parasitic race I made a long time ago based off of the Neurax Worm from Plague Inc. and the Yeerks from Animorphs. None of these had actual established meanings beyond a certain design quality for cards, one off cards for a powerful creature, or basic ideas in a video game/book.

I'm doing this as a labour of love to the Magic: the Gathering game and to the entire fluff of the Zendikar plane within it. Sure it might mean nothing but I absolutely love the worldbuilding they've done and the feel of an encroaching terror or threat can add atmosphere to the campaign and the world it is set in. To be honest I and most people probably won't ever use these massive creature we make, but they're always there and if we feel we need or can use them in a situation we almost certainly will.

Emrakul will probably be used in a form like the Eldritch Moon set: A cult summons her to the plane/world and she begins to cause absolute havok among the minds and bodies of those within the area, needing a group of powerful characters to fight her and seal her away/banish her from the plane (if at best from the other side of the world...) to save everyone that they can and to protect all that they love.