Can you wait and buy it when you arrive? YES
A bar of soap can be used for washing yourself, your hair and your clothing. I know some people are horrified at the thought of not using shampoo and conditioner - but bottles burst in rucksacks and shampoo is a pain when it soaks through everything including the maps, your money and passport!
Try to buy an antiseptic soap rather than a normal one if you can because the added protection is useful when you don't trust the water you are using. Some herbal soaps claim that they can help to keep flies away, and many can actually smell quite nice.
When you've finished using the soap try to let it dry- it doesn't take long in Ugandan sunshine and packing a dry bar is much better than packing a wet one. If you have to pack it wet- slip it into a plastic bag.
Added uses include lubricating squeaking hinges on doors and windows, deodorizing your room, loosening stuck or tough zippers of clothing and rucksacks, and keeping dirt from getting under your fingernails.