Combat, taking a familiar can increase the speed you kill a monster, this will in turn speed up your slayer task and make that illusive 85 slayer just that much quicker, If I take a lava Titan with me in the Chaos Tunnels I can hit a 43 with a whip my titan can hit a 12 and a 5 at the same time, that is effectively a hit of 60, that is the equivalent of a god sword without the expense, a lot quicker and more likely to hit, all the benefit of a GS without the drawback of being a pants weapon and at a fraction of the cost.
The only familiar that has been embraced by many is the Bunyip, this can save a lot of bank trips and makes using Guthans redundant, speeding up training and saving repair costs just by blowing food into your ear healing 2 in the process.
