First of all, as the other answers have indicated, Scalding Tarn does not produce any mana, it just lets you search for another land that can. The colors on this card have absolutely nothing to do with its ability. The blue and red are just there as a visual reminder that the ability lets you search for an Island or a Mountain. The card could be orange, or pink, but the ability would still be the same.
Let's say in your opening hand you have a Scalding Tarn, a Lightning Bolt, and a Spell Snare. Play the Scalding Tarn, then you can wait to "crack" the fetchland until the last second. If your opponent plays a creature, grab a Mountain to cast Lightning Bolt; if they play a 2cmc spell, grab an Island to cast Spell Snare instead. Flexibility!
This is probablyStatistically, the least useful thing to do with"thinning" effect of a single fetchland doesn't make a big difference, especially if you're facing an aggro deckbut multiple copies can. If you play fetchlands and your opponent doesn't, since every pointyou'll have a slight edge in terms of drawing potential threats (instead of lands), but at the cost of some of your life can.
Remember, cards like Necropotence taught us long ago that life is just another resource; using your resources wisely will win you games. That being said, sometimes the life loss associated with fetchlands may not be critical against a good aggro deckwise. AlsoSome tournament-worthy decks run 6-8 fetchlands, statisticallyothers run 1-2, some run none at all. It depends entirely upon your deck versus the "thinning" effect doesn't makemetagame you've chosen to play in.
IMHO, fetchlands are a huge differencemuch better choice when used for flexibility and/or synergy than for just deck thinning alone, but that's just me. You get the deck thinning automatically, but the flexibility/synergy comes with intelligent deck construction.
Anyway, if you don't want those Scalding Tarns, I'll take them. Aside from being good cards for all of the reasons I've just described, those suckers are worth something like $70 each right now!