I searched it on google and got the below result.
a toolchain is a set of programming tools that are used to perform a complex >software development task or to create a software product, which is typically >another computer program or a set of related programs
From the above paragraph, I understood that toolchain or compiler toolchain is used in developing any software product such as OS (I am not sure please correct if I was wrong )
If so, I am presently using Ubuntu 16.04. And I found a compiler toolchain preinstalled on it. But OS is already developed completely so what is need of a toolchain here until and unless a separate software product is built?
It's true that a full toolchain (compiler, assembler, linker, etc) is required to build the OS itself, but it is also necessary for building other software packages. If you download a software package on Linux whose installation instructions say to do some variant on ./configure && make && make install, then you will be using that toolchain to do the compilation. Likewise, Linux (and all the GNU/Ubuntu packages) are developed on a GNU/Linux system themselves, and so will need a toolchain to support that work on the next version of the operating system.