"Sick pay" helps you if you're ever sick for a long time by paying you e.g. 100 Euros for each day that you're unable to work due to illness after 42 days of being sick.
How it works
If you end up really sick, there are 3 phases of "cover" you go through. Sick pay is the "middle one".
Phase 1 (Day 0 - day 42, depending on your work contract)
Your employer covers your pay. Some employers do only 2 weeks, some do up to 41 days. This will cover most sick leaves you have.
Phase 2 (Day 42 - month 6)
Your employer stops paying you. Sick pay kicks in. You receive 100 Euros per day for each day you are ill in this phase. You can use this to cover things like rent, food, etc.
Phase 3 (6 months +)
Long term disability benefits from the government or your job insurance kick in. Sick pay stops.
Summary
There is no general rule here like "always pick it", but in most cases it does make sense. You can of course de-select it.
How to choose sick pay amount
A good rule of thumb is your salary after taxes / 30 - which means that sick pay just completely replaces your salary. So if you earn 3000 Euros netto, then go with 100 Euros.
For most of our customers 100 Euros sick pay is sufficient to cover all ongoing costs, so even if you have a higher salary as long as you don't have expenses much higher than this the default value of 100 Euros is sufficient.