Others have mentioned the importance of joint savings accounts for married couples, and for businesses.

They're also very important for mortgages. If the mortgaged asset is jointly owned (and that's typically the case in a marriage) then you, in practice, need all owners to be a party to the loan. The house is security on the loan - if I only own half the house then I can only borrow against my half. And my wife can borrow against her half. But if I default on my loan, then how does my bank foreclose on me and sell their asset when there is another owner who may not wish to sell.

There are other solutions, but the easiest, and the one that banks will typically insist on is a joint asset requires a joint loan