Re: What effect does Catalase have upon the human body?

Catalase, as you say, converts H2O2 into H2O and O2.  H2O2, and other 
reactive oxygen species, are toxic in tissues (but sometimes beneficial as 
a defense agent) because they can irreversibly inactivate enzymes, damage 
DNA, proteins and lipids or alter the redox state of enzymes.

See also: http://www.graylab.ac.uk/lab/reviews/pwrev.html http://www.accessexcellence.org/LC/ST/bgfreerad.html
and the references therein.

Catalase removes the H2O2 as soon as it forms, thereby protecting tissues 
from damage.  This is especially important during fatty-acid metabolism 
where H2O2 is a by-product.

The implications of H2O2 damage in human health are wide-ranging (see links 
above).  Oxidative damage is thought to play a role in things such as 
premature aging, cancer, atherosclerosis and Lou Gehrig's Disease.  There 
is much interest in preventing oxidative damage with 'antioxidants' such as 
vitamin C, vitamin E, beta-carotene and others which you may have heard 
about in the popular press.