what is the reason why we can get Appendicittis even if we don't know what is the function of the appendix?
There are quite a few things that happen biologically, there is no reason why they shouldn't happen just because we do not as yet know the mechanism or why.
There are quite a few causes of appendicitis, and you might want to have a look at one of the medical web sites for more information. The NHS web site is helpful
http://www.nhs.uk/Conditions/Appendicitis/Pages/Causes.aspx