One side benefit to the title case approach is that references to the button in documentation and elsewhere in the interface preserve the formatting of the element they refer to (without the need for wrapping the text in quotation marks).
For instance, here’s how Apple describes how to set up Time Machine:
1. Open Time Machine preferences from the Time Machine menu in the menu bar. Or choose Apple menu > System Preferences, then click Time Machine.
2. Click Select Backup Disk.
3. Select an external hard drive, Time Capsule, or other storage solution from the list, then click Use Disk.
4. After you choose a backup disk, optionally click “Add or Remove Backup Disk” to add more backup disks for extra security and convenience.
If you removed the title case, all those menu item and button text references would need to be wrapped in quotation marks:
1. Open Time Machine preferences from the “Time machine” menu in the menu bar. Or choose “Apple” menu > “System preferences”, then click “Time machine”.
2. Click “Select backup disk”.
3. Select an external hard drive, Time Capsule, or other storage solution from the list, then click “Use disk”.
4. After you choose a backup disk, optionally click “Add or remove backup disk” to add more backup disks for extra security and convenience.
…and that’s assuming you use verbatim capitalisation inside the quotation marks. Interestingly Apple does use quotation marks for very long or deceptive names in the first example (see the fourth point in the first example), but isn’t forced to use copious quotation marks throughout their entire documentation because of the convention that title case works as a reference inside a sentence without any other special decoration.