A grinder is valuable when you want to adjust particle size and brew soon after grinding. CoffeeBox's equipment guide makes that case strongly, but its claim that specialty coffee is pointless without a grinder is too absolute. Someone can enjoy good preground coffee, particularly when buying a suitable quantity and asking for a grind matched to the actual brewer.
Consider frequency, effort and budget together. A person making daily filter coffee may use an adjustable burr grinder often, while an occasional drinker may benefit first from smaller packages and a consistent recipe. A milk-coffee drinker can also notice changes in coffee strength and quality; milk does not make the base irrelevant.
Before upgrading, identify the problem you want to solve: inconsistent flow, limited grind adjustment, inconvenience or stale-tasting coffee. Reserve money for coffee rather than spend the entire budget on hardware. If using preground coffee, choose a manageable amount, keep it sealed and dry, and avoid leaving it exposed. Fixed minute-by-minute aroma-loss claims in the source are not universal deadlines. The useful goal is a routine you enjoy and can repeat.
The conversation
A good question can make a better brew.