No, it's for having a shared pool of money. Both regularly deposit some amount of money into it, and it goes for shared expenses like food, house maintenance, gas...
I'm going to add that it's also really nice for budgeting. If you want 2 people to get on the same budget you have to show the benefits of the budget.
Also I don't pay 1/2 my mortgage and my wife pays the other 1/2 we pay it all together (from the one account). We actually turned down an account with Plastic because no shared accounts.
We each have an individual credit card but that's more for tracking if we are meeting our own budgeting goals. But those are still linked to the other person.
Also, joint accounts make a situation where something happens to your spouse MUCH easier to handle. If something were to happen to me I'd want my wife to still be able to pay for Daycare, Mortgage, etc for as long as our savings would allow.
This is part of why divorce is hard on so many levels. The paperwork alone is crazy.
Once a business is incorporated, it is its own legal entity and needs to have its own pool of funds that is legally separate from the owners' funds.
There would be one or more accounts held in the name of Super Secret Labs LLC and in most cases that business account would have rules about who was allowed to approve the transfer of funds out of the account, and to what limit. In many cases it may require approval from 2 account holders.
Prior to incorporation, the call on funds would typically be much lower, so it's probably going to be a bit ad-hoc, and might just use an account held by one of the founders with everyone throwing their share when needed. If you can't trust your cofounder with $2k, then don't start a business with them.