People should wear deodorant in order to emit less body odor when they sweat. Deodorant works by destroying the bacteria on the skin that emits odors when feeding on components of sweat.
The odor that a person produces when they sweat varies. Some people smell far worse when they sweat than others, while some barely smell at all. Note that since deodorant works by eliminating bacteria that causes odors, it does not affect how much a person will sweat. There are other products, called antiperspirants, that are designed to limit how much a person will sweat in certain areas like the armpits.