I assume you're talking about OSI-certified open source licenses and the software under them.
There is no obligation to contribute financially, and therefore no justification is necessary. As a user, you're not costing the F/OSS project anything. In a corporation, contributing valuable resources needs justification.
This doesn't have to be negative for the company. If you send in good bug reports, that's a contribution, and many projects also like feature requests. If you send your own fixes or enhancements to the project, that's also a contribution, and if you can push them upstream that's likely to be better for everybody.
Of course, there's no obligation on the part of the project, so if you want any sort of voice in the project you need to contribute in some form. Many projects accept money, some accept contracts, and most will accept developer time and work.
It can be worthwhile supporting F/OSS projects if you can get some advertising or geek cred out of it. O'Reilly, for example, at one time kept Larry Wall (the creator of Perl) on the payroll in the hope of selling more Perl books.