Postcards - because an image describes what you see better than words
Due to texting and social media, we don't send postcards as often now as it is so much easier to forward instantly an image of what you want to show rather than have someone wait to receive in snail mail, but postcards convey the message that you thought enough of someone to actually purchase or print a post, address and attach a stamp and write some expression of emotion without using an emoji.
As a result, it shows that you actually put time and energy into doing something nice for someone you care about. Plus the receipient is thrilled to receive something in their mailbox besides a bill or advert. The fact that it’s a tangible object adds another dimension that email attachments cannot replicate.
They can also act as a personal thank-you card for an act of kindness by a friend or workmate or as a travel diary for yourself. I have collected many postcards from my travels just in case my pictures didn't turn out and have even framed them and hung them on my walls
Fast fact: Did you know that postcard collectors are known as deltiologists.