Eclipse provides basic build facilities, but what differentiates the "pro"grammers from the "amateur"grammers is the greater extent of the professional's skills and toolset, just like the difference between a Boy Scout with a First Aid merit badge and a neurosurgeon. I realize that the popular conception is that any 10-year old kid is supposed to be able to build an "amazon.com" app over a 3-day holiday weekend, but it just ain't so.
One you need more complicated build operations, you need more complex tools. back in the stone ages, I used batch files and the Unix "make" utility, but Ant and Maven are much better suited. And while Eclipse isn't capable of doing builds of this complexity itself - at least without heavy customization - Eclipse does have good support for Ant and Maven. Ant's is built-in, while there's a plugin for Maven.
Another compelling reason for knowing how to use build tools is that
IDE's are far too fragile. I've run into major grief many times because you could only build a project when using a specific version of a specific IDE and it had to have been specifically configured to mirror the local computer system of the person who last worked on the project. Batch tools are less subject to that effect. In fact, Maven's compensation for forcing you to adapt their project structure is that you can download and build almost any maven project with practically zero configuration or knowledge of the particular project being built.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.