Why? Most erratas are not important, but there are always a few which may cause you problems. You really want to double check if the listed erratas are not going to influence functionality of your board.
Why? It speeds up development process. It may be time consuming to start developing software from scratch, especially if you do not have to. It is very hard and costly to develop things like new drivers.
TIP #044: When doing placement under BGA, first only check where the free space is and then do it again
Why? When placing the components – second time, you will know which direction you have some free space.