why is liquid water necessary in the origin of life?

You need some sort of solvent that the various chemicals can combine in. It should be a polar solvent, so that ionic reactions can occur. It also need to be abundant, and liquid over a fairly broad range of conditions. Though some exobiologists have looked at other liquids, such as ammonia, as possible life-enabling fluids, many of them agree that water is the best suited compound.

So we can't say that liquid water is absolutely necessary for life, but it is what supports life as we know it and seems to be the most likely solvent for life elsewhere, as well.

Water can dissolve many substances and does not change temperature very quickly, which makes it perfect for life to be created in.

Water is a very reactive substance, it can start inumerable reactions. Be it heat, or color change. Although water is not required for all life, it is for us because we have evolved to water.