Different types and brands of wire react differently when you work with them. Knowing how each type of wire reacts when you work with it on different projects may make a difference in your choice of metal and wire gauge when creating a new piece of jewelry.
Each type of wire has a different hardness and "spring back." For instance, dead soft copper is much softer than dead soft brass. Therefore, jump rings made with the same gauge wire on the same mandrel with these two metals will end up slightly different in diameter - the brass will be larger than the copper due to the "spring back" of the wire. After trying different wires, you may find that you prefer one brand over another of the same type of wire and/or one type of wire over another.
Tutorials generally give approximate wire lengths because we all have wire working tension, as mentioned above. Because of this, it is a good idea to keep track of how much wire you use constructing various jewelry items.
You probably will not worry too much about the cost of the wire you use when you create your jewelry pieces with base metal wires, but when you start using more expensive silver and gold, you certainly do not want to cut too little or too much wire.
If you keep a chart and/or notes on your projects, you can easily see how much wire you would need for a certain type of project and decide how much wire you would need for your next creation.