What did making the son disabled add to this?

I think it made it easier for the writers to write the series and be able to ignore the son. Can you imagine a normal active late teenage kid being taken in by Walt's antics and weird behavior? I can't. It was all about making a character to create a family, but an innocent family that was not too clueful.

Would have been fun to have Walt Jr become addicted to the blue meth his dad cooked.

I'd be more curious about whether Walt Jr. was written to have a disability before RJ Mitte was cast since he actually suffers from cerebral palsy.

Because why not?

His son’s disability creates another motivation for Walt to accumulate more money. Walt believed that he was a dead man walking, and goes in the meth direction because he believed that he’d need to earn a very large sum of money (in a relatively small amount of time) to leave behind to his family.

Of course, it’s not the only motivating factor, but I thought it was an interesting detail that added a sense of authenticity to Walt’s life in the series. On top of Walt’s mounting pressures, he also has a son with cerebral palsy. Put all of those pressures and details together, and you’ve got a decent man that wants to use his knowledge to accrue a ton of money very quickly.

It brought more awareness to spending time with your family during breakfast.