The ShareAlike clause (or "share alike" in two words; closely related to copyleft) uses Copyright law itself in such a way as to create a code base that cannot be bought-out/subverted by corporations. It allows reuse on the condition that modified versions are shared under the same license or a compatible license.
 Protecting freedom
If something is public domain or under an attribution-only license (e.g. CC-BY) then it's possible for the work to be used in something which is then locked up in a copyright or proprietary work. To put it differently, the public domain cannot resist enclosure.
The idea for "copyleft" as a solution to this was first used by Richard StallmanW in software licenses - see software freedoms. Without this contribution, it seems that we would have BSD-style licensesW (unless some other innovation were made). It seems logical that if all licenses were without a "share alike" clause, this would allow private companies to free-ride on the work of FOSS programmers, increasing cynicism and reducing the motivation for writing software without pay. This would have a drastic effect, especially on the internet (Linux dominates the server market).
Free speech requires free quotation and free comment. A non-copyleft free license such as CC-BY would allow someone to quote you at length and make commentary, but use copyright, so you would be restricted to "fair use" in your response.
The application of this principle in text (rather than software) is more ambiguous. Any original text from an open source is much more obvious (as opposed to software, where the code is compiled into binaries). It is also easier for the end user of text - the one who reads - to access the original and read that. The restrictions imposed by software (usability, compatibility) do not apply to text, and so it is much more difficult for someone to take text, make some modifications and produce a more saleable product. Yet, without copyleft, someone still may be able to create a derivative work and restrict the original author's access to it (e.g. to use the text in a video).
- From the standpoint of proprietary software, Steve Ballmer of Microsoft described the ShareAlike clause as "viral" - a "[…] cancer that attaches itself in an intellectual property sense to everything it touches."
- From the standpoint of extreme openness, the ShareAlike clause has been criticized as blocking a gradual shift to the public domain. See Public domain except as noted.