The annual vaccination prevents against 2-5 of the worst common strains that they think will be in the population that year. It is possible to still catch other viruses & other more mild strains if the flu, but the idea of the vaccination is that you - and any vulnerable people you come into contact with - are protected from the main serious ones.

I get it every year & there have been years when people around me have been dropping like flies - badly ill, bedridden for period of weeks at a time, but I've been fine. I think it's important if you work with vulnerable people (children, elderly, ppl with respiratory illness, aboriginal people, people with chronic illness) that you get it to protect them. But in any other kind of high traffic job (eg, where there are lots of people) or where you travel through airports or work with people who travel, then you are more likely to be exposed to some of those nastier strains too & it's worth getting done