Serious philatelists tend to sneer at first day covers. They regard them as a contrivance of the various post offices and, frankly, a bit of a con to coax money out of gullible collectors.
Now I quite like first day covers. They are pretty, they contain a certain amount of useful information, they evoke a particular era and right now they are very cheap. I just put the New Zealand covers, illustrated above, on ebay. I got £1 for four covers, which I thought was ridiculously cheap. Realistically, I don't think they will ever be worth money, but as long as they remain cheap, they are a nice thing to have in your collection.
see also - British First Day Covers - are they a good investment?