If you had read the bug report and the discussion below it, you'd know that chunks of virtual memory (VM) are not freed to the OS immediately after they are empty, but are only freed after 3 GCs. That's actually an optimization; allocating VM is expensive, and doing it just after you've freed some is plain stupid.
The problem with GCs is not that they don't actually work, it's that they are suited to a specific task. If the user is surfing the web and constantly opening/reloading pages, then GC will be ok. However, if you close most of your tabs and only leave a few open, then you might have to tell the computer that you won't be needing more memory soon and that it's welcome to free as much VM as possible. That's what those buttons are for.
After 20 seconds the browser perform this "idle-cleanup" for you and there is no need to press this button all the time if you want to minimize memory usage.
The problem with GCs is not that they don't actually work, it's that they are suited to a specific task. If the user is surfing the web and constantly opening/reloading pages, then GC will be ok. However, if you close most of your tabs and only leave a few open, then you might have to tell the computer that you won't be needing more memory soon and that it's welcome to free as much VM as possible. That's what those buttons are for.