First, The amount of quest points determine the amount of money that you can transfer from youself toanother player and vice versa, how much you can accept from another player, without going over the trade limit. The more quest points you have the more you can receve in a trade.obtaining Quest Points also increases your Balanced Trading limit. While someone fresh off of Tutorial Island may only be able  to trade with a margin of 3000 gp, you may be able to trade with a margin of 21k! This new Balanced Trading system revolves around your Quest Points. For more information on trade limits, see the

Trading section of the Newcomer Guide.

Quest Points & Trade Limits
Quest Points         Trade Limit
50 or Less             3,000gp
60                             4,800gp
70                             6,600gp
80                             8,400gp
90                             10,200gp
100                           12,000gp
110                            13,800gp
120                            15,600gp
130                            17,400gp
140                            19,200gp
150                            21,000gp
160                            22,800gp
170                           24,600gp
180                            26,400gp
190                            28,200gp
200 or More          30,000gp

Secondly, is to let you do harder quest. Example you need 32 quest points to do Dragon slayer quest .The hardest quest for non members.Also quest points are great when you have done the quest “ tears of guthix” , you go and can free xp from collecting tears from a stream