An additionally guyed tower is a free-standing tower, which is also additionally guyed.
An additional guying can be temporarily or permanently. Temporarily additional guying is used when work on static relevant parts of the tower is done. A permanent guying is attached when the construction has to withstand strong forces in a certain direction — for example, some types of electricity pylon where the conductors change their direction or terminate. Towers carrying horizontally spun wire antennas are sometimes additionally guyed. Sometimes towers in windy areas are additionally guyed in order to withstand increased strain.
The main advantage of additional guying is that it is cheaper than building a completely free-standing tower, which can withstand the same force. Further, it allows for very easy upgrading of existing structures. The disadvantage of additional guying is that it requires much more ground space and that the guy basements handicap the work of agriculture. There is also a danger that the guys can be damaged at their basement anchors, so the basements need to be fenced in.