600 DPI produces much larger required pixel dimensions
At 600 DPI, A4 portrait at 210 × 297 mm requires about 4961 × 7016 px. That is roughly twice the width and height required at 300 DPI, or about four times the total pixel count.
Simply doubling a small source image does not create the same detail as an original image captured or rendered at that resolution.
High-resolution requirements are often used for line art or text-heavy originals
Some line-art and monochrome workflows call for higher resolution than photographic material. The exact requirement still depends on the printer or destination, so 600 DPI should not be treated as a universal default.
Switch to the 350 DPI or 300 DPI calculator when those values are specified.
Higher DPI can also mean much larger files
Doubling DPI at the same physical size approximately doubles both pixel dimensions. Total pixels therefore increase by about four times, which can also increase file size and processing time.
Using the requested resolution instead of an unnecessarily high value keeps files easier to handle.
If the source is fixed, reducing print size can raise effective DPI
The same image has a higher effective pixel density when printed smaller. If you cannot replace the source, a smaller finished size may meet the requirement.
Use Pixels to print size to see the physical size available at 600 DPI.