Ultimately, what purpose is there behind sending Morde botlane?

Honestly.

What does morde have when it comes to being melee and ramp up damage with no mobility, where I could just take bruisers who have consistent and easy to access damage available? Or volibear, a tank who gets damage by building tanky?

If I wanted a kill botlane, why wouldn't I just take some bruisers bot, who, again, have easy access to damage? trundle/xin zhao/jax/pantheon can all crush a lane by being beefy while also having access to damage instantly? Or volibear, who is a bruiser who does have a ramp up for damage BUT ALSO HAS A STEROID TO HELP GET THERE?

You name any botlane support that morde would lane with, and the purpose of the lane, I can name you a champion that would fit that role better, and pull it off reliably.

Or, and just a thought, you could take an ADC botlane and scale much better into lategame, where the ADC role is very important in taking towers quickly.