Selling and delivering a piece of software is not the only work that a software engineer has. Actually the post-developement processes are important too. Maintaining the software is important for a better performance.
Why is performance so important?
Basically it’s important because if there is a software problem that users won’t like it, and if that problem is not fixed then the users will stop using it.
But, even when performance is so important, the majority of fixes during the software maintenance is not for it. Main changes are for software evolution. Time changes and programs should too.
Maintaining a program is part of the software life cycle, but it is not as important as the development part. In my opinion it should be that important, because that process is the one that helps the software to be alive. Without this process our softwares are obsolete and wouldn’t live so long.