In general, the term assignable means that something, usually a property, can be sold or have the ownership be transferred. In business, an assignable contract refers to a contract that allows a specific property to be used and/or profited from even before the deal has been completed.
Assignable contracts can be very useful because a person (or any other entity) who is in the process of acquiring ownership of a property won’t have his/her money tied up without seeing any benefit from it. For example, if a person buys a vacant apartment unit by having an assignable contract that person can start improvements on the unit even before the deal is completed. Once the unit is all spruced up the person can show it to other potential buyers and sell it for a higher price and transfer the contract to the new buyers upon completion of the original deal. This is a tactic used by buy and sell companies to lessen the money they actually have to pay out while maximizing the time profit that can be made from selling a property.
Aside from making a nice profit in a relatively short time, the other benefit of using assignable contracts for cases like these is that the property transfer to your name isn’t finished. This means that you never own that property; therefore you don’t have to go through the hassle of declaring the property as an asset and don’t have to worry about the taxes associated with the property’s total value.