Q:

what is the use of intermediate cert in certificate chains?

What is the use of inter certificates?
I have this certificate chain:

Root-VeriSign Class 3 Public Primary Certification Authority - G5.cer
Inter-Symantec Class 3 EV SSL CA - G3.cer
website.pfx

I know that the public cert of your website needs to be signed by a known CA, not sure why the Inter cert is needed.

A:

a certificate chain is where you have a leaf certificate for your host H, a root certificate in the browser R and R did not sign H directly, i.e.

trusted root R signed intermediate certificate I1
I1 signed intermediate certificate I2
I2 signed host certificate H

To validate H against R you then need to build the trust chain using the intermediate certificates: H <- I2 <- I1 <- R. But since only R is directly trusted and known by the browser you need to give the browser the intermediates additionally to H, i.e. you send H,I2,I1 (in that order!) and the browser than can successfully build the trust path to the known and trusted root certificate R.

