Same as in Norway, and Scandinavian countries. It's the law saying if you live more than half the year in Thailand (or any country), you must pay taxes there. The DTA only makes sure you aren't double taxed.
However, the threshold for not paying taxes on pension in Norway, is 260.000 NOK. Or 23.200 USD, 790.000 baht. It's obviously lower in Thailand, so if that is our income, and what we bring into Thailand, we will pay around 5-8% tax on that, depending on our deductions.
Not much, but still some. And the TIN is definitely needed for us, if we have officially emigrated to Thailand, and reported the change of address.
It all depends on which country you're from, and what your DTA says.