A true balanced line interconnection circuit needs to be present in both of the devices being connected in order to realize any advantage.
For many years balanced line connections were mostly used in professional applications where long cable runs were common. In a recording studio or public sound system environment, cable runs of 75', 100' or more are not uncommon. A balanced line has a much better common mode rejection of extraneous noise, so will be much quieter (less hum, hiss & other interference pickup) with long cable runs.
Balanced interconnects have become a popular option on some higher end home equipment. The trade off is the use of transformers or differential circuits which complicates things a bit. Whether this extra circuitry is worth it or not is something only a buyer can answer for himself.
You are also right there are some pieces of equipment that offer XLR interconnects but do not have true balanced circuits.
Keep in mind there are many other aspects of electronic equipment design that may make a bigger difference than this one issue. In other words, if you use typically short 1 or 2 meter interconnects commonly seen in home stereo use and hear a difference between two components, it may well be due to a factor other than a balanced vs single ended line.
My two cents is don't get caught up with this one issue. If you audition two pieces of equipment and prefer one over another, don't waste too much time worrying about assigning a reason to a particular technical detail.
