It is always important to recognise that when you are dealing architectural landscapes and solutions using Natural Stones (Like Granite, Whinstone, or Porphyry for example) and the laying of either paving setts or paving flags. There should always be a recommendation that there be a joint (or gap) between all of the elements used.
The reason for this being that if you ever drive on top of such hard landscaping materials like natural stone paving there will be pressure and stress put upon the paving sett / Paving Flag which will mean that the stone wants to move. This has to be allowed for, otherwise the paving setts or paving flags will rub off against each other and then will crack on all of the corners. This will then affect both the aesthetic look and the structural integrity of the natural stone paving.
It should also be noted that if they are "butt jointed" i.e. laid tight next to each other this can mean that you get trip hazards or shadows across you’re newly laid paving. This is not the case with concrete man made flags as they have a bevelled edge usually. The British and European standards allow typically a +/- 2mm tolerance on each side of the calibrated cut to stone. The joint allows the paving layer to work with the natural stone and get "straight" visual lines to the paving that will enhance the look at the integrity of the strength of the finished paving.
Similar to the use of joints in facing bricks to allow them to breath while making the facing brick look better, sometimes with the enhancement of coloured mortars and can enhance the landscape design.
There are many types of jointing compound available on the market for hard landscape solutions and these can be researched elsewhere but the common jointing mixture would be sand and cement in a 3 parts sand to 1 part cement mix depending on the strength required. Again these would be set out in the British and European Standards Guides.