My experience as a long-time Googler has been that people do 20% projects for two main reasons:
1) They want to change their role and view the 20% project as a means to an end. As an example, I worked with two 20%'ers that wanted to transfer from non-SWE to SWE roles. One of them has already succeeded, and another is hoping to be able to do so as soon as head count permits.
2) As a labor of love. As an example, we have an epitaphs page containing information about/farewells from people that have departed Google. It's not an officially supported project but is still quite popular. These kinds of projects don't necessarily lead to promotions (although the criteria for promotion have recently changed), but on balance they make people's lives better.
1) They want to change their role and view the 20% project as a means to an end. As an example, I worked with two 20%'ers that wanted to transfer from non-SWE to SWE roles. One of them has already succeeded, and another is hoping to be able to do so as soon as head count permits.
2) As a labor of love. As an example, we have an epitaphs page containing information about/farewells from people that have departed Google. It's not an officially supported project but is still quite popular. These kinds of projects don't necessarily lead to promotions (although the criteria for promotion have recently changed), but on balance they make people's lives better.