By some search and research I find it is logically known as to be fried evenly for the whole donut, otherwise it may be fried and expands like a irregular rugby football.
1. Donuts
To make donuts, the maker has to either remove the center part of the paste, or connect two sides of a paste rod.
This reminds me a quite similar food in China, at least in my hometown.
2. Fried rice cake ( Face-Concave 面窝，ps: 窝 for concave )
It is very popular in Hubei Province. It is one of the most famous food in Wuhan and the cities around it. Actually it is one of my most favorite choices for breakfast during my primary school when we are allowed to have breakfast outside of the school individually.
The making of the "Face-Concave" is much easier than making donuts -- the maker do not need to bother the shape at all. They have special designed spoon to produce the desired shape.
The only thing they need to do is to fill up the spoon to some extend, as long as one can see the bumpy center of the spoon, and put the spoon into the boiling oil. These special spoon can save a lot of time and efforts of the breakfast makers.
I guess one the reason they design such spoon and the shape of Face-Concave should be similar to the one of making donuts -- to fry them evenly.
In fact there is an even simpler shape and making of food which has a long history. (recorded among 1127—1279AD South Song Dynasty) :
3. deep-fried dough sticks (油条）
Unlike 面窝，this one is popular almost all over China. You can see it in almost any city and county in China.
They are always made in such a way that a pair of sticks attached to each other.
The social reason is that people at that time hates their "Prime Minister" couples so much that they decide to make a snack symbolize the PM couple, fried them and eat them every morning...
The physical reason is that, for a single stick it is very difficult to fry it properly. The oil temperature and the time spending on frying must be rigorously controlled otherwise the taste and appearance will be very lousy:
Someone has done some experiments with single, double and triple sticks.
So besides the social reason, the current popular making of fried dough sticks seems like an optimized result. The expansion caused by chemical reaction which releasing lots of CO2 to make the sticks fluffy.
