Sure a lot of Thai girls are good in English especially hookers. But how if you want to ask a “normal” Thai girl out for a date? There are countless of beautiful Thai girls that have never dated a foreigner before but would love to if you can at least speak some basic Thai.
Or think of all the university girls most of them don’t speak English or are too shy but how if you tell them in Thai “You are cute, do you want to go for dinner with me?”. Imagine how many more Thai girls you can meet if you know some really useful Thai vocabulary and phrases!