@Phil *****
you only get the 12 month extension at the immigration office INSIDE thailand.
I think you're asking why people would go to Savannakhet to get a year-long multi-entry Non-O VISA from there when the requirements are the same for a year extension in country.
Some people work rotational work and can't be in country the amount of time it takes to burn down a 90 day Non-O visa to where you can apply for the year extension, then wait the 30 days under consideration period, then go back to get the stamps inked in. It's easier for them to bounce in and out on a year long multi-entry visa with 90 day stamps