The analogRead will be a little more noisy. Using a number of samples and use the average can solve that.
Sometimes the AREF is connected to the 3.3V pin for 3.3V analog sensors.
The AREF can be used for other things as well. When the internal reference is set, the AREF can be measured with a multimeter to check what the exact internal reference is. That means it can be used (with many restrictions and pifalls) as an output.
It is even possible to use AREF as some kind of analog input (with many restrictions and pitfalls), since AREF can be measured with the internal reference as reference.
When a external voltage reference is used to AREF, the analogReference() has to be called before analogRead(), or else an internal shortcut will occur.
There is an internal resistor from the AREF pin to GND of about 32k.
You wanted to know exactly what the AREF pin does, check the figures in the datasheet. However, it is impossible to tell exactly want can be done with it, because there are probably more and even worse ways to misuse the AREF pin :o