At the time of their invention, railroads were huge for industry. We could now transport a lot of heavy goods quickly across the state and country. We needed someone to be in charge of the railroads, trains, and their regulations.

Nowadays, the railroad commission doesn’t have jurosdiction over rails in TX, instead they have primary regulatory jurisdiction over the oil and natural gas industry, pipeline transporters, natural gas and hazardous liquid pipeline industry, natural gas utilities, the LP-gas industry, critical natural gas infrastructure, and coal and uranium surface mining operations.

https://www.rrc.texas.gov/about-us/

I think the idead behind it being an elected position, rather than an appointed position, is to mitigate corruption. If the citizens see corruption taking place in the department, they can elect a new railroad commissioner in a few years that will set it all straight.