Season 7 Placements - Why more placement matches?

Why do we need to do more placement matches? I had a message popup saying that I'll start season 7 in Silver V. However, per the FAQ https://support.riotgames.com/hc/en-us/articles/231832207

> Even if you place during preseason, you’ll become unranked again in all queues once the 2017 Season begins. In order to earn a ranking for a queue, you’ll need to play ten placement matches in that queue.

Why do we need to do 10 more placement matches if they've already determined where we'll start??

What about the TT flex ranked queue? Why bother having the 10 placement matches here only to have them all reset again when the season actually starts?