In many cases it may be required that you need to boot your phone into recovery(in this case your S2). It can be cases such as you have applied a pin lock on your phone and you forgot its password. Other cases may be you want to just format your your phone or wipe the cache partition which cannot be done directly via the android OS, you have to do it manually.
Unlike the galaxy ace which is the mini version of the SGS2 you can’t just hold the home button and power button to boot into recovery, you need to press something else too. Lets check out—>
To simply boot SGS2 into recovery press and hold the home button+volume down button+power button. Its not over yet, as soon as you see the samsung logo, leave these buttons and after a second, press them again. You will straightaway boot into recovery. And thats how you do it.