What would be the benefit of giving intravenous albumin to a patient who has experienced fluid loss and has low blood volume?

When fluids like saline are administered IV they equilibrate with the total body water. That is, most of the fluids leak out of the vasculature and wind up in the interstitial space. Albumin is known as a "colloid" solution and mostly stays in the vasculature (very little leaks out) subsequently raising oncotic pressure and actually sucking in some of the fluid from the rest of the body. The point of giving albumin is to replete the person's intravascular volume without causing fluid overload in the rest of the body. This can be important if someone is massively fluid overloaded as in liver failure or heart failure, but they don't have enough of that fluid in their blood vessels to maintain an adequate cardiac output.