I agree that no one can predict the future. However, we can make better decisions if we know probabilities.
For example, if you really research a stock you can find out that maybe it never dips more than 25% in a year in a bull market. And maybe the average dip is 15%. In which case if you see it dip 15%, then buying it after a 20% dip means you're likely to make money if you hold it one year.
So knowing certain stats like that can help you make better decisions on when to buy.
Another thing you can do is look at support and resistance lines and fibonacci retracements. If you know it dropped to support, then there is some probability it could bounce off of it. There are other things to consider, but, that's just a generalization.
Or maybe the fibonacci retracement is never more than the golden pocket of 61.8%. And so if it drops to there and you buy, you have a good chance it's going to go up soon.
You can also look at moving averages and buy when a stock is well below key moving averages.
Finally, I like to look at the oscillators like the CCI and the RSI. If you see both of those at the bottom, then again, you have a good chance it's at or close to a bottom. It can keep going lower, but, again, it helps. It reduces the probability that you're buying at a top.
So, if you're a long term investor, using some of these indicators can help you make a good timed purchase and it could definitely help you increase your returns over the long run vs not looking at these at all when buying.
Just a warning that if you start looking at these too much, it gets addictive and you may have a hard time being a long term investor :) So, don't look at these too much unless you want to trade vs invest.