No, Bitcoin is not used for investment purposes.
In the past, many have said the same.
Bitcoin has no usecase, it is to slow and expensive and is hardly used for anything.
Buying bitcoin is no investment because it has no usecases, it is a pure ponzi scheme. It cannot technically be classified as an investment.
It is an investment and that's why many people are getting their own portfolios with bitcoin. It is not a ponzi scheme, a ponzi scheme guarantees you profit but bitcoin is volatile and that's its nature asset that you have to decide whether to invest or not. Being expensive, no use case and slow. Well, investors won't be getting in and holding it if those descriptions you have mentioned are existing on it. About it being slow, segwit is making it quicker.
Now you hold BTC for the money you gave to the exchange. When the ponzi scheme ends, you lose most of your money. This is not investing.
You hold the money that you have. What if you didn't bought from exchanges and you just mined it?