In the field Through normal use, kite lines eventually develop weak spots. Always keep an eye on your flying line as it goes out or comes in. Be on the lookout for frayed spots, cuts, pronounced kinks, surface abrasion - any damage that could weaken the line. Cut these out then and there and re-join the line with a Blood Knot or Double Grinner. Never leave sections of old flying line in the outdoor environment.
Line is chosen on the basis of both the size of the kite and the wind it flies in.
You need light line for small kites and heavier line for big ones, and for any given size you need light line for light wind and heavy line for strong wind. An assortment of lines is essential for flying in a wide range of winds. The force the wind exerts varies with the square of the wind speed. With light line you'll get the steepest flying angle and the best possible performance in light winds and thermals, while heavier lines give more steadiness and stability in breezier conditions. 1,000 feet adds up to over 5 square feet of surface area, which equals a fair bit of drag in addition to its shear weight.