I have used the term big fish rather than specimen fish on purpose. A two pound chub is not a specimen chub but it is a big fish and will put a good bend in your rod when you hook one. There are several important reasons why legering is such an effective method for catching the larger species of fish. Most of the larger species feed on or near the river or lake bed. Barbel, tench, bream and, to a large-extent carp, are all bottom feeders and this is where it is necessary to present the bait. All these fish will occasionally feed nearer the surface and carp will often accept a surface bait, but for consistent success the bait has to be where the fish feed most of the time, and that is on the bottom. The most important reason why legcring for the larger species of fish is so successful is that it allows the presentation of a bait which is extremely attractive to the big fish but is immune from the unwanted attentions of the smaller ones. To do this it is unnecessary to use a really large bait, rather one which cannot be nibbled away by the small fish.
The tackle used has also got to be strong enough to withstand the powerful fight put up by the fish when it is hooked. This is especially true if you are fishing water which is full of weed or sunken trees which you must prevent the fish reaching. Presenting a large bait using strong line on float tackle has lots of problems, especially in a flowing river. Long float rods are not designed to be used with strong lines and a rod of 12 feet (3-6 metres) designed for float fishing is not powerful enough to drag an angry barbel away from a sunken tree root. The stronger nylon line is, the thicker the diameter and the less flexible it becomes. When using a stronger line, bait behaves in a less natural way because the nylon is not flexible enough.
Legering overcomes this problem because the line near the hook lies on the bed of the river or lake and is not affected by the current. In a fast flowing river, especially one that is fairly shallow, it is not easy to present a bait acceptable to big fish on float tackle. Barbel and chub will not often chase a bait dragged through at speed with the current but will accept an offering trundling along the bottom using a rolling leger.
Legering also enables you to anchor a bait in places such as a small gap between overhanging willow branches on the opposite bank of a river which are impossible to reach with a float. In a lake or pond it is acceptable to present a large bait on float tackle at close range. Use an antenna float attached by the bottom end only and fish over depth so that the bait is well on the bottom. Fishing at long range, however, does require the use of leger tackle for ease of casting. There are no hard and fast rules to determine where and when to leger for the larger fish, but try and take into account the weather and water conditions. To consistently enjoy catching the larger species of fish, have an open mind and do not restrict your fishing to one method or bail.