Your standing rigging is there to support your mast and hold it up high into the sky. To do this, the stays need to be strong enough to withstand the loads and also setup at the correct angles to properly transmit these loads through the yacht.
The minimum angle of a stay approaching a spar is 12 degrees. If the stay approaches the spar at an angle less than 12 degrees, the stay will not be able to exert the needed force on the spar to resist movement.
Lowers are able to travel directly from the chainplate to the spar without any guidance because they approach at a wide angle, greater than 12 degrees. The further up the mast you go, the lower the angle would be and the less effective the stay would act.
To fight this problem, spreaders are used to hold the stay out, allowing it to rise up vertically and then turn towards the mast, reaching it at an angle of at least 12 degrees.
This same engineering tacktic can be seen on other areas of boats. Long bowsprits will have "Dolphin Strikers" which are spreaders for the bobstay, as well as spreaders for the whisker stays. These are all there to help achieve the needed minimum angle of 12 degrees of approach between any spar and stay.
