It's a milk formula for a 9 month baby. Are there any scientific reasons requiring to boil the water before mixing in the formula's powder? The baby already drinks the tap water as it is healthy for human consumption here in Switzerland
No, you need not boil the water, especially as your child is already nine months old.
The recommendation to always boil is just to be extra safe in reducing bacteria content. Note that even very clean tap or bottled water is not sterile and neither is water that has been boiled for a few minutes. In general the recommendation to wash / boil everything you give to your baby becomes quite futile around six months: the imune system should be quite developed and a child that crawls will put much more things (starting with the hands...!) in his mouth than a smaller infant.
However, you should always use fresh, cold water, never warm tap water, which may have higher contents of bacteria, microorganisms or heavy metals.
Simply warm the cold tap water to the temperature recommended by the formula manufacturer. Discard any leftovers.