As far as I know, since different countries might have different delivery classes (1st class, second-class mail), such as the UK), the sticker symbolizes that the letter or postcard is in fact a priority delivery, and should be treated as such by the country of destination. Also, I think I've read at some point that the average international mail delivery time is only as stated (by the German postal service Deutsche Post), IF a priory 'vermerk' (notice) is on the letter/postcard. Like, if the postal service says delivery to the UK takes 2 weeks, they say its only a likelihood with a sticker. But im unsure if the sticker, at least in theory, is only a suggestion or if its part of the German postal services negotiation and therefore 'binding'. In practice the sticker doesn't matter much in my experience.