Thai is more of a lifestyle-language, like Italian or Swedish. Thai is spoken by approximately 50 million people, mostly in Thailand. The exact roots of the Thai alphabet are unclear, but it is believed to have come from Southern India. Thai is a tonal language, meaning that there are five different tones used, sometimes the same words can have different meanings based on these tones.
Being able to speak Thai can open up the world for you in Thailand. The people will flock to you in admiration. Many Thai people are shy, but they will be attracted to you if you can converse in Thai. Even if they know some English, they prefer to be able to fall back on Thai, especially if they want to hang out with their friends.
It's also good for business, even if you are in an English environment. You will have a better idea of what your staff and colleagues are thinking and be able to get your ideas across more directly and accurately than to rely on bilingual go-betweens. If you work in a school or environment where it's all Thai then you won't be so isolated and will start to make friends amongst your colleagues and their friends more readily than before.
And you will be surprised at how much more cheaply you can live in Thailand, once you can read and communicate in Thai.