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.
