1 If you never rebalance, you're never adding new stocks to the index, nor removing stocks that do not belong to it anymore.

2 You need to rebalance to take corporate events into account: new stocks, buybacks, dividends, etc...

You can add stocks whenever you put money in. Whether that's because you got your paycheck or a dividend or some other income is kind of irrelevant. And you can remove stocks when you take money out. But you probably shouldn't start selling one stock to buy another just because their prices moved, unless you have information that lets you time the market.