This is just a guess, as no one knows for sure what mystery cards mean, but:
Will there be some sort of reward if you have this card later?
Yes, probably something to do with Summer Sale, which will begin any day now.
Will it be worth more later?
No. It's a simple law of supply and demand - the more of these cards are on the market, the less they are worth. Price has already dropped from 2,5 to 1,05 Euro.
Only if Steam suddenly stopped mystery card drops (limited supply) and then made them really worth your while (increased demand), then the price would soar. But that's a slim chance, personally I'd bet on the first option, that the price will gradually drop.