Sure... Well, I don't have a specific reason:
1. I'm interested in minority languages.
2. Back in Jr. High School and High School (that's 2002~), I used to watch many Singaporean movies, and many of them had some Hokkien on them. I remember I got really interested in Singapore (I even used to listen to Singaporean bands!), but unfortunately never got to visit the country. The only book for learning Hokkien I knew was "Spoken Hokkien", but it was useless because I couldn't afford the tapes and I knew learning a tonal language without audio would take me nowhere. A friend of mine used to joke that Hokkien was great to swear... (Is that some kind of cliché joke?) I know, however, most speakers are eldery.
3. Again back in High School (around ~2005), one of my Japanese teachers was learning Chinese. I then asked her if it was because she was dating a Taiwanese, and she told me no, 'cos at home they spoke Taiwanese. Too bad I didn't research anything about it, because otherwise I'd find out many of 五月天 (MAYDAY) songs were actually in Taiwanese, and perhaps that would have boosted my motivation to buy those expensive textbooks and tapes? Who knows...
4. About one year and a half ago, a friend brought me to a Taiwanese association. As we got there, we got approached by an old lady who asked us if we were Japanese. She was very kind to introduce everything there, and she told us about Taiwanese language, all that in a nearly perfect Japanese. Before we left, she even gave a Mandarin textbook for each of us. After that, I started reading about Taiwanese language and found out it was actually Hokkien (I was like wow!). On the same day, I learned 五月天 had Taiwanese songs (and now I was like wow wow wow). So I made up my and decided to learn Taiwanese. I didn't have enough money for textbooks, but we were only a few months away from Christmas. So I got "Maryknoll Taiwanese" vol. 1 and 2, "Harvard Taiwanese 101", "The Little Prince" and "おれは鉄兵" DVDbox, but I was a little disappointed 'cos all text in the lessons was romanized and dumb me thought hanji on the glossary was Mandarin translation. Harvard wasn't really good for self-study 'cos there are no grammar explanations (if any, they're all in Mandarin). So I put them aside (okay, I'm lazy!). Then again, around Christmas last year, I decided to give a try some Japanese textbooks for learning Taiwanese. I got them around February, and it's been about one month since I'm working through "CD Express Taiwanese". After that, I'll probably try "New Express Taiwanese" (the newer edition by the same author, but a completely different textbook), and then "台湾語会話" (I still missing the CDs) before trying Maryknoll and Harvard.
That's just a bunch of unconnected reasons, but basically that's why I want to learn Hokkien. I want to learn Taiwanese first, but later learn the differences with Amoy (and perhaps Penang) and how to avoid Japanese loans in order to communicate with other Hokkien speakers as well.
And when learning, I really need hanji, even if it's just some random character. The point is, I completely fail to recognize words in an all romanized text... Perhaps it's because of my tone impairness?
As to why I've chosen Hokkien over Mandarin... Well, I do languages for hobby, so usefulness isn't important unless I really have to learn the language for a specific need. And personally, I don't like Mandarin retroflexes (like those -er sounds or R as in 溫柔). I'm prepared to get bashed for this comment

I'm not saying Hokkien is better than Mandarin, nor the other way around, just that Hokkien sounds a lot better to me and that counts a lot when choosing a language to learn...