There's a few possible reasons

1. If the person is laid off (the position is made redundant) there may be Government mandated compensation for that person. For example 1 week of pay for every year worked. This is to protect workers from being laid off on a whim, the company can't just let their workers all go without any penalty.

2. Some companies have contracts that offer severance pay to long time workers if they are terminated. It's a bonus of working there, and is meant to thank them for long time service and help them find a new job. Often this is a Union benefit.

3. Often unused vacation, pensions, and other bonuses accumulate and have to be paid out if the person is terminated.

4. High level Executives are notorious for including clauses in their contracts that they must receive payment based on the remaining time on their contracts if they are fired. These are referred to as Golden Parachutes and are the result of executives getting to effectively write their own contracts. So even if a manager fails at their job catastrophically, a company may be forced to 'buy them out' of their contract to be rid of them.