What is the role of cobbler in PXE booting technology?

In the simple words - Cobbler is just a boot server, which helps in installation and deployment of systems.
With Cobbler you can in few steps configure instalation, serve DHCP for machines (and create profiles with defined system per every MAC in local network), and so on.

hi to PXE boot, you need a DHCP server to hand out addresses and direct the booting system to the TFTP server where it can download the network boot files.
you can use the manage_dhcp from cobbler to do that.
also when you have setup your cobbler, when you edit your system it makes a sync of your distrubutions, except services (dhcp etc).
to answer in generall what is the difference between cobbler and standard pxe..
cobbler is simpler.

What can cobbler do for you in bare metal installation is it can manage entire PXE tree for you, static/policy driven which means if you have situation that you can't do PXE "cobbler buildiso", it has also built-in PXE boot loop prevention and auto generated PXE menus for "off the truck" deployment.
It is simply a web based deployment systems.