MACD and Bollinger Bands are two of the most widely used technical indicators, but they measure fundamentally different things. MACD tracks momentum by comparing two exponential moving averages (typically 12-day and 26-day), generating signals through crossovers. Bollinger Bands measure volatility, plotting a moving average with bands set two standard deviations above and below it.
Traders compare them because momentum and volatility are two separate dimensions of price behavior — MACD tells you about the strength and direction of a trend, while Bollinger Bands tell you whether current price is stretched relative to its recent range.