TAGs that are visited often are valuable in the real world, becuase you can advertise or offer prizes on them. However, they are subject to attack and takeover by another player to control the advertising or information on it. To attack another TAG, you must own the closest TAG to it, which will become the attacker. Click on the TAG you wish to attack and from the info screen click the red ATTACK button. It will attack with 100 Points every 20 minutes, giving time for the owner of that TAG to defend by adding more points to it. If you are the owner of a TAG under attack, you will get a notification - to defend, click the green ADD button on the info screen for that TAG, and it will add 100 points, deducted from your balance. Each attack of 100 Points will reduce the defending TAG level by 50%, so for example if a TAG has 100 power level to start, it would require 2 attacks to take it over, providing the TAG owner does not defend it.
If you find yourself under attack, you can also counterattack with the defending TAG, to try and take over the TAG attacking you, with the same rules as above.