Building your own computer can sometimes be worth it and sometimes not depending on your needs for a computer.
If you're just needing a new computer for general use such as browsing the internet, checking emails etc then you can usually just go online or into a store and pickup a cheap desktop computer brand new for around $300.00 to $400.00
In that case it's not really worth it to go through the trouble of sourcing the parts and building your own computer unless you just want something to do and want to build the computer yourself.
However sometimes if you want to build a more powerful computer such as for video editing, gaming etc then sometimes building your own custom computer for that purpose can be worth it.
However sometimes you can find a good power gaming computer for a $1,000.00 or $3,000.00 right off the internet or in a computer store.
I've built a computer before just for the experience and fun but for my use it was actually cheaper for me to go get a new computer off the shelf of Walmart.
Because it cost me $300.00 more to build the computer with the same specs as a cheaper Walmart computer.
It was a learning experience though and fun so that part was worth the extra costs.