@Sarah *********
It's to make life easier. I have a non-O "retirement" visa (It's actually a visa based on being over 50 years of age - there's no requirement to be retired), but I travel a lot. I'm probably in Thailand for 7-8 months of the year, the rest of the time I'm travelling. However, rather than live in Thailand on a succession of tourist visas, or free entry stamps (which annoys the crap out of immigration officers), I simply have the one visa (or actually extension) which does me for the year. I always extend about six weeks before expiry, so if there's any sudden need to leave, I'm covered to leave and return. It's great flexibility