There may be various reasons why co-owners choose to separate their shares in their property.
This may be for the simple reason that one co-owner is putting in more money than the other to the purchase of the property and wants to ensure that they receive those funds back in the event of a sale.
Alternatively, one party may be paying more towards the mortgage and the bills as ongoing costs and therefore they may wish to state that they own a greater share of the property than the other party.
The relationship between the co-owners will often dictate how the equity is divided. For example, friends or business partners will likely want to specify their shares in the property so that they can ensure this is left to their own family members in the event of their death.