1. For a party at investor's expense
2. To encourage important people to feel that they've got to support the venture in future
3. Knowing standards of work in this country they need to have time to repair all the things that were done wrong in the first place
Maybe these are a bit cynical but I've had a bad day and I'm taking it out on AB.
Usually the soft opening is to work out the kinks, acclimate the staff, and gauge public reaction on a smaller scale so any adjustments can be made before the grand opening. It's like a Broadway show in out-of-town previews - correct the production before the big-money public sees it.
A building project is in the hands of a Project Manager until final commissioning is completed and then handed over to the owner. It is the owner who organises an official opening. The owner usually seeks out some notable - eg. a politician or the Mayor - to perform the official duty. Such protocols take some time to put into place. Then it's party time!