Most times it's much cheaper to just buy a new computer from the store or online than it is to purchase all the parts and build your computer from scratch.
I build my own desktop computers when I need too because I enjoy the building process of the computers and love shopping for computer parts.
I can select and install quality computer parts in my computer and give it the computer specs I want it too and need in it.
If you want to save money then just buy a computer but if you want to source the parts yourself and love building a computer then build it.
But you usually won't save any money although you'll have a quality built computer if you do it right.