@Samu Hi, just want to chime in again on this. Yes, a dpi is a dot of ink on paper. A ppi is the digital approximation for screen usage. 300 dpi (interchangable with ppi in Photoshop) is the file setup requirement to print an image sharply. Laser printers will actually print mono images up to 1200 dpi. But it's not really a dot from a laser printer. Digital laser printers fuse toner onto paper, they don't lay down dots. But that's another topic...
The dpi and the physical size (in inch/cm/mm) work in unison to form the image size. For screen display, pixel size rules. For print usage pixels AND physical size need to be considered.
But offset printing uses 300 dpi, and can have different results depending on who is printing it. Proper offset printing is absolutely an art. Proofing images and print files on screen is very common now, and is one of the reasons finished printed artwork is sometimes not as expected. All screens display colour differently. Screens are set to display at different resolutions. As well as different printers having different skill sets and level of care - sometimes printing skill doesn't match actual printed colour proofing sheets.
I'm actually a little surprised to find that working in RGB is so common for illustrators. I'm a graphic designer (+15 years industry experience), and we work with CMYK images for print. We convert RGB image files to CMYK, adjusting the ink balance as required for different printing methods and substrates. Having said that though, photographers work in RGB...
It is only quite recently that pre-press operators have been happy to accept RGB images, as the rip software they use is now able to interpret the RGB colour profile quite effectively.
I can only assume that it is because RGB is monitor colour, so you get a 'true' example of what you're creating as you are creating it. However this then isn't 'true' when converted to CMYK for printing - or even on another screen. The only other reason I can think of is that the software has dictated this - in that some effect tools and lighting/layer adjustments only work in RGB colour mode.
By using the 'save for web' function out of photoshop you are using the technology as it was designed to optimise a 'print useage' digital file for display on screens/web. I have done extensive testing on this output method and found that it is the best way to ensure crisp web graphic assets. It is not ideal to set text in Photoshop (rather in layout programs which create vector edges); this is not the case for some online advertising requirements where we are forced to include text in Photoshop.
I've gone a little off-track here, but it is relevant to understanding file setup, and how to setup for use across different media.