To reduce load on the web server JpGraph implements an advanced caching system which avoids the burden of always having to run the full image script.
Depending on the complexity of the image script (for example if it is doing several DB lookups) this could significantly improve performance.
The rationale behind this is of course performance, and the observation that very few graphs are really real-time, i.e. needs to be updated absolutely every time the graphing script is called.