1.) Its generally every other major patch. (Tier 1 was a few weeks after 6.0 launch, tier 2 was 6.2, final tier will be 6.4)

1a) Because the tier is around 8 months long.

2) Its more or less a catchup mechanic I guess. You don't have to have someone in your static pre-clear to skip and you can just jump right into the fight you're trying to prog late into the tier.

3) The same reason other MMO's make you run through the entire raid to get to each boss after the weekly reset? FFXIV doesn't really do old school raid designs anymore where you run through basically an oversized dungeon and clear trash before each boss fight so they have to create some kind of time sink.

4) It honestly doesn't. You can clear the entire tier in the base level crafted gear. I personally prefer it didn't work that way and was more traditional MMO style that actually required progression where you have to upgrade your gear to meet the dps checks in later fights.