[R-SIG-Finance] Capital requirements as a cushion against risks, why?

[Copula Guy]

I am currently programming different VaR models with R and I often read about the Basel framework. There the capital requirements are determinded by a fomula which uses the VaR. Now I am wondering, why a bank needs to hold a certain amount of capital? 


In general, it says, the minimum capital requirements are used, to 
determine the capital that banks must hold as an insurance against risk.
E.g. if a Bank has invested in one equity asset, e.g. Microsoft stock (long). If now the stock drops, the bank's asset looses value - so the 
bank has a loss, but it does not have to actually PAY something? Why 
should the bank hold extra money as an insurance against this drop in 
the assets value?

Although this is not directly a question about R programming in finance I was wondering about this, when I did my R project.

Thanks a lot in advance!