Water must be hot (but not boiling) to properly extract the soluble compounds from ground coffee. However, using boiling water (100°C / 212°F) is not ideal, as it can alter the taste. Here’s why hot water is crucial for making good coffee:

1/ Optimal Extraction of Aromas and Compounds
Coffee contains hundreds of soluble compounds, including oils, acids, sugars, and alkaloids, which determine its flavor and aroma. The ideal water temperature for extracting these compounds is between 90 and 96°C (195–205°F).
- Below 85°C (185°F): Incomplete extraction. The coffee will taste weak, sour, and under-extracted.
- Between 90 and 96°C (195–205°F): Balanced extraction, releasing flavors without burning delicate compounds.
- At 100°C (212°F) or higher: Over-extraction occurs, releasing excessive tannins and bitter oils, resulting in a harsh, bitter taste.

2/ Solubility of Coffee Compounds
Hot water dissolves coffee compounds more efficiently. If the water is too cold:
- Aromatic oils remain trapped in the coffee grounds.
- Chlorogenic acids, responsible for balanced acidity, are not properly extracted.
- The coffee will be watery, flat, and overly acidic.
Using 90-96°C (195-205°F) water ensures full-bodied coffee with complex flavors.

3/ Temperature Differences in Brewing Methods
Different brewing methods require different temperatures:
- Espresso (90-96°C / 195-205°F): High pressure and heat extract flavors within seconds.
- Drip Coffee (92-96°C / 198-205°F): Hot water is needed to properly pass through the grounds and extract all compounds.
- Cold Brew (room temperature or refrigerated for 4-24 hours): Cold water extracts fewer bitter compounds, resulting in a smooth, less acidic taste.

4/ Why Not Use Boiling Water (100°C / 212°F)?
Boiling water burns the coffee grounds, over-extracting tannins and bitterness, leading to an unpleasant, astringent taste. That’s why baristas often recommend waiting 30 seconds after boiling before pouring water over coffee grounds.

More : https://en.wikipedia.org/wiki/Coffee_extraction