Many times after an extraction or more generally when there is a lack of a posterior tooth, many patients are asked the question: Why should it be restored since it does not look? Why should I destroy next strong teeth by preparating them as I can eat and talk with the rest? Unfortunately for the majority of patients only anterior teeth deficiencies need immediate treatment because of the direct aesthetic deformity that is created. Restoring the absence of the posterior teeth is just as imperative as the anterior teeth. This is because they also participate in the proper functioning of the mouth. However, even when the justification for postponing prosthetic restoration is the destruction of intact adjacent teeth an alternative is the placement of dental implants, so we do not affect residual dental tissues.
When we make a bridge or place an implant, we do not seek to restore only the aesthetic of the mouth but also the proper and smooth functioning of chewing, speech and swallowing and try to avoid the general disorganization of an environment in which the teeth are in equilibrium forces (Fig. 1).
Fig.1 In the normal situation the position of the teeth is determined by the forces between them.

Fig.2 The created gap changed the  balance of the system.

Fig.3 The opposite tooth was led to hypereruption.

If we leave a gap in the mouth without replenishing it, then the result will be the adjacent teeth to be moved(fig. 2), the hypereruption of the opposite teeth (Fig. 2 and Fig. 3) and thus disorganize the mouth. The same can happen even with the loss of the integrity of a single tooth so the opposite erupts until it comes into contact with the damaged tooth (Fig. 4).

Fig. 4 The destruction of only part of the tooth led to hypereruption.

Fig.5 The teeth due to a lack of adjacent ones have gone out of the chewing level (ME).

Fig.6 The teeth have almost come in contact with the gums of the opposite jaw.
The teeth that are located next to or opposite the point of a missing tooth tend to fill this void in a way that creates a problem because they either interfere with the normal movements of the mouth (Fig. 5 and Fig. 6) or the aesthetics (Fig. 7 and Fig. 8).

Fig.7 Large interdental intervals due to backward displacement of the teeth.

Fig.8 Revelation of the root of the teeth due to their excessive sunrise.

Fig.9 Moving the tooth forward and around its longitudinal axis.
It is clear that any attempt to replenish the absence without the dentist’s contribution will result in abnormal tooth movement (Fig. 9) and weakness, as long as it is sufficient for proper restoration (Fig. 10). In this case, such a preparation should be made so that the bridge can be inserted (fig. 11), while considering the case of implant placement it is obvious that this is impossible due to lack of space.

Fig.10 The inclination of the tooth changed the parallelism of the longitudinal axes.

Fig.11 Attempt to parallel with a suitable mode of manufacture.

Fig. 12 The deflection favors plate retention and charges the periodontium.
Also, an inclined tooth causes damage to the surrounding gums due to abnormal transfer of chewing pressures (Fig. 12).
Concluding, we would say that replenishing a tooth that is lost is done to:
A) restore the balance, function, aesthetics and comfort of the patient
B) maintain the physiological state of the remaining teeth
C) prevent new lesions
That is why there is no question of destroying the adjacent teeth, but on the contrary, these teeth are guarded by abnormal movements and contribute, as a support, to the smooth operation of the whole mouth.
Hence the construction of a bridge or a crown is not only a therapeutic but also a preventative process, since it prevents a series of undesirable effects that if we do not deal with them in time, we will lead to the disorganization of a set which by its nature works harmoniously.
However, even if the patient does not accept the preparation of the adjacent teeth for bridge construction, there is the alternative that he hears in the name of dental implants.