The sympathetic nervous system's primary process is to stimulate the body's fight-flight-or-freeze response. It is, however, constantly active at a basic level to maintain homeostasis (Brodal, 2004).
The heart rate and breathing increase is needed for homeostasis due to the need for more blood oxygen and fuel by the muscles.
##References
Brodal, P. (2004). The Central Nervous System: Structure and Function (3 ed.). US: Oxford University Press. pp. 369–396. ISBN 0-19-516560-8.