Some employers ask it because they are hoping for either two things to quickly happen. They either want to prune you out, such as if you are experienced from elsewhere and demand more money than they want to pay, or they want to identify you as a lowball offer with too little funds.
It is sometimes a sign of a bad company, but also sometimes a symptom of bad or aggressive HR departments at an otherwise good company. I consider the tactic a bad warning sign, they are more interested in cost rather than quality, meaning there is a higher likelihood of treating people like cogs, to be replaced by the lowest bid. It isn't necessarily true, but it does mean you should be cautious.
I'd dodge the question for now.
One option: "It is too early to discuss salary. We both first need to decide if we are a good fit for each other, then we can talk numbers."
Another option: "Since this is an international job search, I haven't done all the number-crunching in your area. How about we first see if the job is a good fit for each other?"
Another option: "I'm looking at several nations and haven't computed your city's costs yet. How about you tell me what range you are in, so I can factor that into my local cost of living calculations?" (Note that they will give you a lower number if they give you any range at all.)
If they're looking for a deeper reason, explain that you have not even interviewed with them yet, you don't know the details about the company, you want to be sure you can do the job well before discussing any kinds of compensation.
In salary negotiations, usually the first person who mentions a number loses the negotiation, either the worker getting less than the company was willing to pay, or for the employer paying more than they had anticipated. There are many books written about salary negotiation. An employer asking early about expected wages is an oft-mentioned 'dirty trick' used by bad companies, but also sometimes used in ignorance. For best results you should wait until after they have decided they want to hire you, and you have decided you want to work with them, before you mention any numbers.