What is the use of relational database in Graphite when there is whisper database library?

I am using Graphite to monitor my services. I send real time metric data to Carbon-cache which then stores data points using the whisper library into a .wsp file.

I believe graphite has a default graphite-web app built using Django that reads data from the .wsp file and display it in form of graph ? Then why does it use a relational database does it save the data points in it or does it save the graph the admin creates ? I don't understand the use of a relation database here because graphite saves data into a .wsp whisper file so why do we need a database ?

Also I have a nodeJS app that sends data to graphite every 5 minutes works perfect!. However, i want to send past historic data too so i wrote another simple nodejs app to sent 2 months of past data to graphite it went in fine i can even see data but what happens is when i say show me last 2 weeks data on graph i can only see the data that was sent from my node app that send every 5 min not the historic past data that was sent from my other mynodejs app and vice versa? does it have to do something with the way i have set my storage schema ? How do i send past data cos i cannot afford to run it every 5 minutes since i have almost 5000 records like that which will take me couple of day ?

/etc/carbon/storage-schemas.conf

[sqldbypassigned ]
pattern = capacity\.assigned$
retentions = 5m:1d,10m:30d,60m:3y