Re/oblivious > Oblivious
Essentially yes, release dates are actually very important to the success of media releases, be it music, videogames, films etc. You've got to take into account all manner of things like national holidays (Christmas being the big one in the West, although other countries may well have similar scaled events with similar effects), plus the fact that people are more willing to spend money at certain times of year compared to others. On top of that, there's also the issue of competition as everyone's trying to do the same thing to get in on those seasonal bonuses.
Releasing a single or an album even a day or a week later or earlier could coincide with a release from a far larger, more popular group which can easily overshadow your sales and push releases down on the charts, if not outright force people to purchase either one or the other if they happen to be fans of both acts. Sadly Kalafina is not even close to the popularity of numerous other acts, so if their single release happens to fall within the general area of some of the major artists it'll hurt their ranking potential and sales.
As for anime tie-in singles, those will naturally come at pretty regular times each year as anime release schedules are based on a seasonal system, generally with one cour per season and often one opening and ending song per cour. Of course, it makes sense to release the single whilst the anime is still airing as that's when a considerable amount of people will be watching. I've seen a lot of series over the years and I can't even remember the openings or endings for most of them, so if they released their songs right now I likely wouldn't even pay attention to it unless it was an artist I'm already a fan of, whereas if they released the singles during airing (and most likely did at the time) when I was watching maybe I would've picked the CDs up.
Theoretically at least, I rarely actually buy singles other than Kalafina. Hell, the only other artists I've even downloaded singles for are ClariS, Aimer and Perfume despite liking a lot of other artists. I do know some people who almost exclusively buy anime theme singles however, so that's some evidence that the system works.
