"...it is very hard to keep track of what needs to be built" - Build tools does not help with that all. You need to know what you want to build. (Quoted from Ritesh Gun's answer)
"I heard they're used almost in all kind of real-world development" - For some reason, software developers like to work in large companies. They seem to have more unclear work directives for every individual working there.
"How come I never needed them in past four years". Probably because you are a skilled programmer.
Pseudo, meta. I think build tools do not provide any really real benefit at all. It is just there to add a sense of security arising from bad company practices, lack of direction - bad software architectural leadership leading to bad actual knowledge of the project. You should never have to use build tools(for testing) in your project. To do random testing with a lack of knowledge of the software project does not give any sort of help at all.
You should never ever add something to a project without knowing it's purpose, and how it will work with the other components. Components can be functional separate, but not work together. (This is the responsibility of the software architect I assume).
What if 4-5 components are added into the project. You add a 6th component. Together with the first added component, it might screw up everything. No automatic would help to detect that.
There is no shortcut other than to think think think.
Then there is the auto download from repositories. Why would you ever want to do that? You need to know what you download, what you add to the project. How do you detect changes in versions of the repositories? You need to know. You can't "auto" anything.
What if we were to test bicycles and baby transports blindfolded with a stick and just randomly hit around with it. That seems to be the idea of build tool testing.
I'm sorry there are no shortcut
https://en.wikipedia.org/wiki/Scientific_method
and
https://en.wikipedia.org/wiki/Analysis