It's not your employer's business if you're going to hospital for the day to get your extremely embarrassing medical complaint fixed, no. However, it is certainly the company's business if you're off sick.
Employee absence is usually a company's largest client. It's factored into all calculations so the fact that more hours are spent on absence than anything else doesn't matter particularly. What does matter is the reasons for this absence.
If, for example, this is the 20th time that Employee A (Bob) has called in sick this year then it's something that a decent company will look into. Not to punish Bob but to ensure that he's not overworked or to see if there's anything the company can do to help. Depending on the country you're in there may be statutory sick pay after a length of time off, which means that your salary will be accounted differently.
If you're on maternity or paternity leave you may get some statutory pay from the government/your employer which needs to double checked and applied for by payroll. Maybe the company won't have to pay any tax on your salary.
If you're on annual leave the amount of days you have remaining need to be calculated.
If you're asking for bereavement leave a larger company will normally have a maximum amount you're allowed to take in any one year. Whilst this may be flexible the amount remaining will still be tracked.
If you're volunteering somewhere, your company may have a specific amount of days that you're allowed to do this for. In the UK, this means (I think), that the company can write off a portion of the work done against tax.
etc.
Simply put, the reason is not about you specifically but about HR and payroll (and the tax man) keeping on top of the varying methods of payments that make up your salary and the government's take of that. Or, keeping track of the amount of allotted days you have remaining for activity X.
There's no need to go into details of why you're taking sick leave, or annual leave. Just to state that you are - that's all the information needed in order to process your leave in the specific manner dictated by the type of leave requested.