Bleeds are required when images or colours run to the edge or edges of a page, these images or colours need to overlap the edge of the page so there is something to trim into.
If you do not have bleed on your document you will get uneven or white edges when it is trimmed around.
One illustration overlaps the crop marks, that is with bleed and the other is without.
When setting up your document make sure you set it up as the right size (do not set documents in the middle of an A4 page e.
g.
a business card).
If you are looking to save time and money by laying out the document yourself, check the requirements with the printers you will be using, otherwise you maybe wasting your time and they may need to charge you for resetting the document, any images will need to be CMYK.
When you submit your document to a printers they will need to impose it (multiples on a sheet) which is another reason why you need to have bleed for objects running to the edge of the page.
Tip 1 Check what size your document will need to be with your printers, Some documents are a specific size for a reason, for example business cards are 88x55mm the reason for this is it makes it the same size as a credit card so it fits nicely in a wallet or card holder.
Tip 2 Make sure any images are CMYK unless you are including spot colours.