I know I have explained this somewhere else

but let me try here...
dpi is a derived measurement - it means dots per inch. The reason we choose the magic number of 300 dpi is to match our printer capability and get the best quality printed page we can. I've tried at 200 dpi and there is a difference. I've tried higher, and there is no difference - at least to my eye.
I make pages that are 8 by 8 inches. At 300 dpi they are 2400 by 2400 pixels. Now the number of pixels is important since your printer prints dot by dot, or pixel by pixel. If you have any less than that the printer [and software] compensates. Perhaps they have more dots of the same colour or it guesses what the colours should be in between - either way you will most definitely notice this at 72 dpi if you want your layout to still be the original size.
It is not a good idea to start at 72 dpi. With my 8 by 8 inch layout this means the number of pixels is 576 by 576. It might look good on the screen but so few pixels on a printer is going to make my page look very disappointing indeed.
Hope this helps. Remember - it is total number of pixels that counts, not dpi.