Thailand Immigration is notorious for being jerks to visitors and a lot of the time it seems as though they are doing it just because they can. When you are in an Immigration line it doesn't matter if you are a street-sweeper or the CEO of a company, the person on the other side of the glass gets to treat you like shit if they want to and unfortunately all too often in Thailand that seems like the de facto way of doing things. I'm always on edge when I leave and especially when I enter the country.
I've always found the arrival cards to be rather stupid pieces of paper that there is no way anyone ever actually reads that crap, but in an effort to, I guess save money on paper, the arrival card process has been made digital now and of course as is the usual for Thailand, they never told anyone about it but they will rip you a new one if you didn't do it.
It's the same information that you would have put on your usual hand-written arrival card that used to be passed around on the planes 20 minutes before you arrive and everyone would be scrambling to borrow a pen from a stranger but now there are more questions that no doubt, are never looked at by anyone.
Here is the link to the website. If you do not do this before you arrive you will still be allowed into the country but not before they treat you like a child, berate you, and take you off to a special room to berate you even more for something you probably didn't even know existed.
Gone are the days where you just got on a plane and traveled to Thailand to have a good time like it was when I first arrived here. No, now this nation seems to be actively searching for ways to talk down to you while at the same time basically begging the population to please please please come and visit this country and spend money PLEASE!
I talked to a friend of mine who flew over to visit me and I neglected to tell him about this because it simply slipped my mind. He said that by not having this that his entry into the country ended up taking roughly another hour because it wasn't enough to just talk down to him, they had to overly scrutinize all the information that he entered in and when he couldn't remember the address of the hotel that he was staying at they were reluctant to even let him use the wifi in order to find out what it is. They definitely could have simply looked it up for him, but nah, that would be far too helpful and friendly.
Eventually they gave him the wifi password, which is odd to me that airports have wifi passwords anyway. What are people sitting outside leaning on the building stealing the internet for free? Aren't we past those days?
Anyway. Just know that this is a very real requirement in order to get in the country. Sometimes they don't ask for proof of it, sometimes they do. If you don't have one done you may get lucky and get through anyway but just to save yourself some hassle just go ahead and do it. It will only soak up around 5 minutes of your day. Here is the website again if you need it.