The only reason not to buy your equipment before your OW course is monetary-- the risk that you either buy something that isn't right for you (jacket or back-inflate BCD, jet fins or split fins or other brands, etc) or you may choose not to continue diving.
There are many good reasons why having your own equipment for the OW would be a good idea. Getting your own BCD is a good idea if you are an odd size. If you don't fit in the usual S/M/L sizes and usually find yourself in between sizes or need XXXXXL or XXXXXXS, then there is a high chance the rental BCD won't fit you well. If it doesn't fit well, you will find that it is more of a hindrance and you will be struggling during the open water dives.
I think the gear that most affects your first diving experience would be the mask, fins, and maybe BCD. If you get a mask that doesn't fit and it keeps flooding, you're not going to enjoy the dive. If you keep getting blisters from the fins and are flailing about, not going to enjoy it either.
So if cost is not a problem, go ahead and get what you want. No harm, just get the shop assistants to help you find the right fit for you. Important to buy at the shop if you don't know what you need.