Part of the reason for advocating widespread flu shots is that no disease is particularly ¹dangerous if only a few people get it. Depending on how easily communicated a disease is, there is a certain "critical abundance" of infections that must be exceeded for that disease to spread rapidly, become widely distributed, and to infect large numbers of people - epidemic/pandemic style.

¹ except to the few who get it.

If a sufficient part of the population can be kept resistant to something like flu, even if the shots don't give full immunity, the ability of a new variant to reach the critical level of infection to spread rapidly is significantly reduced.

Case in point is the polio pandemic of a few decades ago. Fairly benign variants of the polio virus were (and are) well known, and infection with them maintained a base population with widespread resistance, including some resistance to the paralytic form. The Swiss got indoor plumbing and started washing their hands (in a national campaign for civic improvement). The benign variants dropped out of sight. The paralytic form reached the crossover point and rapidly infected the whole world.

Individuals may make their own decisions about whether they need to get flu shots. Those at high risk of getting whatever flu is circulation probably would want to. For a few, the shots may carry special risks, and they should not. The need is to keep a reasonably large percentage of the population as a whole as resistant as possible, in the hope that a general and widespread resistance to "new flu-like illnesses" may help prevent a new pandemic.

Of course, there are other theories too.

John