Does poker equity still affect us when we fold?

One thing which I don't understand with poker equity is that when you calculate equity is it per street? For instance, say on the flop I had 30% equity against my opponents range and I called the bet with the correct pot odds, would that 30% equity just be for the flop or for further streets? Because as we know in general you can narrow your opponents range depending on how he plays on further streets etc;

What I don't understand though is that if we had 30% equity on the flop ie 30% chance to win at that point, and we end up folding on the turn, haven't we just given up loads of chips pointlessly, as we haven't won yet or haven't technically lost the hand mathmatically either?

The way I compare it to is from a bet on a coin flip. If we had 50% equity on a coin flip we are going to know one way or another whether we are right or wrong after the coin is flipped correct? We either win 50% of the time or not, but in Poker we don't always get to see the end result of a hand? anyone getting this? Like you may have 30% equity on the flop which is great with good pot odds, but if we fold on the turn haven't we just wasted our chips away making the equity calculation useless?

I hope someone can follow me on this.

You can't keep calling when your equity starts to fall no matter what the math says. Bad players chase too many draws and weak pairs all because of equity percentages. Doing so will cripple your stack and influence how you play the next hands, and not for the better. I fold more draws these days than I ever did early in my poker playing days. This is due to not wanting to put my all my eggs (chips) in one basket with a hole in in it. The hole being the likelihood your draw will bust. You have to raise with draws and weak pairs instead of just calling, raise for information and if you steal the pot, well that's even better. Get away quick from your weak holdings or raise to steal.