1. allows to queue dungeons of that expac, otherwise you can only queue bfa dungeons. you can still walk in without it but cant queue.
2. puts you on a different shard on the server that will not have any level 50+, since level 50+ cannot use chromie time. so there is less chance of others farming a rare that you need, or turn warmode on and not have level 50+ killing you.
3. for higher levels, allows the expac to scale to level 49. e.g. classic normally scales up to level 30, with chromie time it will scale up to level 49 to allow you to continue gaining xp past 30. doesnt apply if you are level 30 or below, and different expac have different normal cap