What is the main benefit of the Thule one key system, what does a Master Key do and how simple is it to change the locks?

A common myth is that when you purchase a Thule product you only receive one key. That is definitely not the case, as you always receive two keys. In reality, the one-key system allows you to conveniently match all the locks of each product to the same key number. For example, you may have a set of roof bars, a couple of bike racks, and a roof box all having different key numbers which can be a nightmare to organise. This is especially so since the locks do not have the numbers showing.
In reality, the one-key system allows you to match all of your locks to a single key number. For instance, you may have roof bars, bike racks, and a roof box.  As you can guess, having different key numbers may be quite troublesome, especially when all the keys look the same to the naked eye! 
There are a couple of ways of achieving this. One option is to purchase a set of four, six, eight or twelve locks and keys, along with a master key for removing the old locks. Ideally you would specify a lock number that you already have. You can then replace each lock on the remaining Thule products so that they all match each other.  The most cost effective alternative is when you purchase a product from a Thule merchant, such as roofracks.co.uk, who offer the service of supplying all Thule goods with specific lock numbers upon request and without any additional charge. This means, if you want to add a couple of bike racks to your roof bars, the bike racks come with locks and keys that match the roof bars.

Thule master key                            Thule 450600 lock and key set

Finally, another misconception is the use of a Thule “Master Key”. Its name suggests that it will magically unlock any Thule lock if the original keys are lost. Luckily this is not the case as it would be a thief’s dream tool. The Master key’s purpose is to remove a lock barrel from the lock housing and to do so the lock must have been unlocked with the original key. It is not possible to remove a lock barrel with the lock locked. Once the Master key is inserted into the lock you simply withdraw the key with a little bit of a wiggle and, as if by magic, the lock barrel comes out attached to the Master key.
When removing a lock barrel from a Thule roof box the procedure is slightly different. The lock is not completely unlocked before inserting the Master key as is the case with other products. The original key is inserted and rotated just a quarter of a turn backwards and then removed before inserting the Master key and removing the lock barrel.