It's called compounding interest, you buy it today for $1, and over time it's value goes up to $10.
Now, when you buy at a regular pay period, you want them to be as cheap as possible, buy low, sell high.
You obviously can't time the buying in a 401k, but you can shift it around a little, --though when the market goes up, you want to be along for the ride. I don't shift.
When you retire, your average cost will be cheaper then what you sell them for, if the market plays out historically, and if not, my Dad say's there won't be anything left to do with your money anyhow.
Just get enough to be sure to get all companies matching funds, that's the first part of getting a return. I wouldn't go over that limit, if you want to save more take it and go invest in something else.
I got 27 years before I can retire so, because my time horizon could ride out the lows in the market, I go for the higher risk funds too. When I'm 50 or so then I'll think about being a little conservative.