I've never heard of CityPay.io before, but it sounds like something similar to Binance Pay or other payment processing services.
Why would Tether invest in such services when it can expand by purchasing rechargeable payment cards such as Visa and MasterCard, and with their stable currency, it may be an attractive option for many investors?
Probably due to the restrictions from regulatory frameworks so they needed a company which was already there to achieve what they wanted, just assumption though. Still it paid off for their investment that is why they are interested in expanding further.
Just as every other company does have partners, I also believe that's what Tether offers; they don't need to be directly involved in the company, but their funds are working for them, as they also have some legal agreements with the Georgia government on their need for expansion in the country.
This is also the first time I have heard about CityPay, but from what I just read in the article, they have already been in existence since there are a large number of active payment processors, which is a great audience, and they believe now is the best time to launch other services that can aid the company more, like the crypto card they propose and the rest of them. Currently, I think they are the leading crypto payment processors in Georgia, so Tether probably has foreseen their investment return in the near future.