I am not that good at German, but correct me if I am wrong, 'entwerten' is to do with validating your ticket on public transport. When you look into the meaning of the word, it is 'devalue'. I was wondering why it is de value? Surely you are adding to it's value?
I agree with Carsten's comment. If you validate ("entwerten") your ticket you cannot use it any longer after the trip. It has lost its value because you won't be able to sell it to someone else. Note that "to validate" does not mean "to add value" but "to make valid" (for a certain trip), so there is no real contradiction between the English and the German word, just different points of view.
entwerten can not only be used in the context of tickets. It is much clearer when you use it in the context of money. Money can - due to inflation - lose its value; it "devalues". Das Geld wird durch Inflation entwertet.
About anything that has a value can be entwertet. For example if you ask me to explain something to you, what I say has a certain value to you, because you trust me that I know what I'm talking about. If later someone can prove that I was wrong, this entwertet my answer. The value of my answer becomes zero.
The use of
entwerten for validating a ticket for public transport actually stems from the separation between time of purchase and actual use. in many transport networks a ticket is only valid for some period of time after entering the transposrtation facilities, therefore the law-abiding commuter is expected to have some machine print a time stamp on the ticket before embarking.
after that, the ticket only has value for a limited time, which motivates the use of
de-value for the process.
obviously, the connotation of the term no longer match the technological state-of-the-art and many kinds of fare calculation and billing.
The answer is simple. A ticket is like a voucher. Or a coupon. It has a value. This value can be traded in. In this case for a trip.
Coupons or vouchers you usually give to somebody so you cannot use them twice.
With tickets you usually keep them, because there is nobody to give them to. But in order to avoid multiple use of single use only tickets, they have to be de-valued or destroyed so they cannot be re-used. That can be done via stamps or punching holes into them.