The so called "visa runs" are for getting around not having the appropriate visa.

I was on a proper employment visa 8 years, never needed to leave the country.

No, you can extend your visa at the local immigration bureau if you have the appropriate type (that lets you work, though even tourist visas are extendable¹) and pay the cost².

People who do visa runs are doing so to extend their tourist visas for cheap.

¹: I've done it before at Si Racha Immigration Bureau. In my case it was more expensive than doing a visa run or paying the fine for 3 days of overstaying, but it was less hassle and left a positive/neutral mark on my passport, not a suspicious one.

²: This can sadly also require bribery just to get them to do their job, even if you do everything by the book. People around us were bringing bribes in form of food. We weren't asked for a bribe, probably because the person I was traveling and applying with had an official passport.