Haha. Basically, in my experience for shows which end up having a number of seasons or large casts (i.e. opportunities to put out character songs), certain types of shows end up amassing a fairly large collection of songs such as OPs/EDs or character songs. After building up such a music repertoire, it probably then makes a lot of sense to ask the seiyuus or performers to hold a live concert or music event, in order to promote the property or bring in some extra revenue. In shows in which the music/seiyuus are particularly prominent, these concerts seem to even be used as incentives to promote BD sales, by providing things like early access vouchers for ticket pre-orders (very common) or recorded footage of past concerts (as I believe is being done with some of Love Live's BD volumes).
The particular Saki event I'm aware of was held sometime last year I think called Saki Fes, a torrent recording of which is still floating about online (I actually happen to be one of the only two seeds remaining for it actually
, though that's mainly because I haven't found the time to sit down and watch through the whole thing yet). I believe that there are also youtube rips of a few parts of it which have been posted online; lemme look up some links:
If you'd be interested in the whole thing, I can pass the torrent link over to you via PM (I don't recall exactly, but I'm not sure Animesuki allows freely linking to other torrent sites). I will note that it's altogether about 3 hours and 45 minutes of unsubbed footage though (split over 4 parts), and a pretty large file (BD quality, haha).