Why would I need to get a tetanus booster?

Increase antibodies. Over time antibodies wane from previous tetanus immunization, so a booster is needed to increase the level of protection against the disease tetanus.
Protection wears off. The protection provided by a tetanus shot usually provides ample levels of circulating antibody protection for up to ten years. After that point it fades & the person may develop tetanus if exposed. Protection from tetanus & several other bacteria based vaccines does not last as long as most viral vaccines & must be boosted periodically to maintain protection.