The majority of people who have hyperhidrosis seem to find their sweaty palms to be the most bothersome problem. In general, sweating in other areas of the body is easier to tolerate in comparison to the hands. When I used to sweaty heavily in my palms, feet, armpits, face and chest when I was in my late teenage years, the palms were by far the most bothersome problem. The vast majority of human eccrine sweat glands are in the palms and soles of the feet.
While the feet are usually covered with socks and shoes, the hands are always exposed and frequently in contact with other humans and various objects throughout the day. If you have excessively sweaty hands, it becomes a pain to shake people's hands, to write with a pen without smudging the ink, to play racket sports without have the racket slip, to sew, to cut vegetables, to tie shoelaces, and much more. Many people become somewhat reclusive because of these difficulties.
While heat and stress can often exacerbate one's sweaty palms, the actual problem comes and goes even when one is calm and the weather is moderate or cool. Hyperhidrosis of the hands results from an autonomic nervous system disorder that is usually involuntary in nature.
As with all types of sweating, there are numerous hyperhidrosis treatments that one can attempt to reduce or even cure excessive hand sweating.
The below image shows excessive perspiration on someone's hand. Such wetness and moisture can clearly be quite uncomfortable and distressing, both functionally as well as in social settings. Simple daily tasks such as writing, typing, driving, cutting, turning doorknobs and more can become torturous.
The International Hyperhidrosis Society has much more information on sweaty palms and related concerns.