Right after roasting, beans are packed with CO2, a natural byproduct of the roasting process, trapped inside the bean's cellular structure. Brew too soon after roasting and that gas pushes back against the water during extraction, disrupting how evenly the coffee actually gets brewed. The result, no matter how good the beans are, is often a flat or sometimes sour cup, with the coffee's actual character muffled underneath excess gas escaping mid-brew.
Most specialty coffees hit their best window somewhere between five and fourteen days post-roast, once enough CO2 has degassed for water to extract properly and evenly. Push much past that window, and the coffee starts losing its more delicate aromatics. So freshness still matters, just not in the way most people assume.
Brew method changes the ideal resting window too. Espresso generally wants more rest than filter coffee, because the pressure involved in espresso extraction is far less forgiving of trapped gas than a slow pour-over. Naturally processed coffees often want a touch more rest than washed coffees, since the extra density from dried fruit sugars traps gas differently. None of this is a hard rule, it's a starting point worth testing with your own gear and your own taste.
The practical upshot: if a bag was roasted yesterday, that's not a reason to celebrate, it's a reason to wait. Buy coffee with enough lead time that it can actually rest before you need it, and check the roast date the way you'd check anything else with a shelf life.
It's also part of why we push roasters to print roast dates clearly rather than burying them, and why "freshly roasted" claims deserve a second look rather than automatic trust. A coffee's peak isn't the day it comes off the roaster, it's whenever the gas has settled enough to let the flavour underneath actually show up in the cup.