I did the lottery thing once while in Tokyo. It was for a Sheryl figure that had 3 slight color variants. Paid 6k yen, drew a ticket, and got one of them. Luckily I got the color I wanted on the first try. The figures themselves aren't for sale at normal retail shops. Have to get them secondhand from places like Mandarake.
If I recall, there are different prices for different prize levels as it would be hella lame to pay 6k yen and get one of the cheap 100 yen prizes.
As for why Bandai does this... well, making things available for a limited time usually boosts sales. Ask McDonald's about the McRib to find out more!
Did you get the Sheryl figure that was released alongside that Freyja?