Firstly, welcome and congratulation to you for starting magic. I'm particularly curious why are you interested in extended cards rather then standard cards?
Never mind about that. In magic the gathering "Land" cards is one of the most important cards needed to create the deck especially land cards with extraordinary ability.
Players who uses Land card are able to gain significant advantage over players who does not. That is why special land cards with ability are particularly expensive but it is pretty worth is if you are a full-time player.
Special lands card such as flooded_strand are able to excel your playing field by letting you gain momentum, tempo and possible changing the tide of the game. There is also unlimited combos where you may use the land card to full benefit.
Take for example, combining flooded_strand and knight of reliquary (standard card) http://sales.starcitygames.com/cardscans/MAGONS/flooded_strand.jpg
With only a reduction of 1 life you are able to:
- Increase your creature power/toughness
- Gain Tempo by having extra mana
- Trim down your deck ( so that the probability is higher to pull out better cards "top decking")
You may also abuse creatures abilities such as landfall, unearthed and etc..
There are practically unlimited combo that is unaccounted for using just this card, thus that is why the demand is high and expensive.