There are multiple reasons to get a Thai tax ID. (1) If you need it for work or other purposes, (2) If you hope to have your bank no longer withhold the 15% tax on interest (for the first 20k baht or whatever) and presuming your bank will honor that request (as some posters say has occurred), or (3) If you want to file a Thai income tax return to receive back the mentioned tax from the Treasury.
I obtained a tax ID years ago and filed a couple of tax returns and got the withheld 15% back when it was worth bothering with it (haven't bothered in the last couple of years because the interest withheld wasn't much but may do it again in January as you can claim back 3 years worth).
Would note that I attempted (2-3 years ago) with my tax id card to ask my bank (SCB) to stop withholding the 15% and they simply said "no can do." Maybe I'll try again.
