You do not have to be an expat to invest offshore, but you should take advice, preferably independent, before investing.
Why invest offshore?
The main reason is that your investment will normally accrue interest free of local taxes. This is not say that it is Tax Free - you will usually be liable for income tax on the money when it is repatriated. However, if invested for a reasonable period of time, the accrual of interest should be greater in a tax free environment than the tax eventually levied - at least, that's the theory.
It is essential to take advice before investing offshore. Because of the expenses involved, new bank accounts and the (usually) higher charges on offshore investments, it is usually not worth it for smaller amounts - again seek advice.
Citizens of some countries (notably the US and France) have government restrictions placed on them as to where they can invest; usually only into approved funds.