In theory, investing is not gambling because the expected outcome is not random; people are expecting positive returns, on average, with some relationship to risk undertaken and economic reality. (More risk = more returns.) Historically this is true on average, that assets have positive returns, and riskier assets have higher returns. Also it's true that stock market gains roughly track economic growth.
Valuation (current price level relative to "fundamentals") matters - reversion to the mean does exist over a long enough time. Given a 7-10 year horizon, a lot of the variance in ending price level can be explained by valuation at the start of the period. On average over time, business profits have to vary around a curve that's related to the overall economy, and equity prices should reflect business profits.
The shorter the horizon, the more random noise. Even 1 year is pretty short in this respect.
Bubbles do exist, as do irrational panics, and milder forms of each. Investing is not like a coin flip because the current total number of heads and tails (current valuation) does affect the probability of future outcomes. That said, it's pretty hard to predict the timing, or the specific stocks that will do well, etc.
Rebalancing gives you an objective, automated, unemotional way to take advantage of all the noise around the long-term trend. Rather than trying to use judgment to identify when to get in and out, with rebalancing (and dollar cost averaging) you guarantee getting in a bit more when things are lower, and getting out a bit more when things are higher. You can make money from prices bouncing around even if they end up going nowhere and even if you can't predict the bouncing.
Here are a couple old posts from my blog that talk about this a little more: