In answer: imagine a standard Lab / Tory two-way fight seat. But you are a Green. So they go 1 on your list. 2 for you is Lib Dem – you hate both the main parties. If you only listed 1 and 2, your vote would still be wasted in all probability. But using all the separate numbers means you could vote SWP for 3, Independent for 4, Lab for 5. So you would be continuing to vote AGAINST Tory until the last possible stage. If your 1-2-3-4 all get eliminated your vote will STILL prevent the Tory you don’t want getting the seat. Had you just had options 1 and 2, this would not have happened – your vote would have disappeared.