Hey, usually most printers require you to add a 3mm bleed to business cards and leaflets before printing. So if your designing an A5 leaflet which is 148x210mm, your finished design to the printers needs to be 156x216mm.
If your using photoshop, you can add this by just making your canvas size 6mm higher and wider. Stretch any images and backgrounds to the end of the border and bear in mind that all this will be cut off.
The bleed on your leaflet gives the cutter some room for error (most leaflets will be printed litho on huge sheets of paper and need to be cut out afterwards). The paper itself can expand or contract, the cropping machine could setup wrong or the person working on the leaflet could make a mistake. There are a lot of factors that could go wrong with the cropping, if you wouldn't be using bleed the images wouldn't be neatly aligned with the side of your printed document
You also need to set the colour scheme to CMYK and make sure that the resolution is in 300dpi. CMYK gives the best view of what the colours will look like once printed, and is the correct colour setup for printing.