Generally the hot pass is made with higher current and the weld area is hot after making the root pass. The root pass will generally have greatest dilution with the base material and as a result will often have the highest Carbon Equivalent (hardenability) in carbon and low alloy steels. The second pass provides some tempering of the root pass and provides greater thickness to the weld to mitigate cracking, especially when welding might be discontinued and resumed (filled) later or when external applied loads are present. Also the added thickness provides greater protection from burn-through when fill passes are made with considerably more heat input like when submerged arc welding is used for the fill.

The hot pass will also push the root pass into the pipe. Often when the root pass is completed it will be basically flush with the inside of the pipe. Once the hot pass is laid down over it the root will be pushed into the pipe and you will see the "wedding ring" band where the weld protrudes slightly into the pipe.

Regards
Stonecold

The hot pass is generally made so if you have to walk away from the weld for a period of time ( taking a break etc.) it will eliminate or at the least reduce the chance of your weld cracking if and when it cools down. They are also referred to as sinkers - "Welding a sinker in"