In a lot of circumstances, people are most willing to give things to charities that they can spare a lot of. For instance, a lot of people donated tooth paste to the victims of the catastrophe in Haiti a few years ago. And while tooth paste was very useful, both for oral care and for keeping down the stench of sickness and death, it can actually be very difficult to package up and transport the items that people donate to a charitable organization. After all, when a charity takes things, those things must be dealt with individually. Often times, it is best just to donate some cash to a charity- not because they are greedy or will use the money for anything shady, but just for the logistical ease of being able to use the money to buy what needs to be sent over.
Far too often, people donate things which are either difficult to transport or in irregular quantities. While a few bars of soap might do some good, imagine what you could do if you could simply raise enough money to buy a few bars of soap, instead. While it might sound like the act of comparing six to half of a dozen, the differences can actually be rather extreme. Think for a moment about how much you might be able to help someone, just through donating cash.
Cash is a universal method of buying whatever the charity deems that the affected people need the most. In some cases it is beds. In other cases it is medicine. It will almost always involve clean water. Since a charity is often able to buy these items at a reasonale discount and transport them in massive quantities to the necessary location, it can be a definite boon to them for you to simply give them the money to buy them. A dollar in their hands might end up buying more than two dollars in yours.