Buying stock is trading future money than money today which is a real and meaningful trade. On the other side, I might want a new car, which means I want money, which means I want to sell stock.
Even stock to stock transitions can be meaningful as Bill Gates had a lot of MS stock and wanted a hedge so he sold stock. What he got was probably worth 'less' the diversification was valuable to him making the transaction a net positive.
What you are talking about are precisely the aggregate benefits to the markets I mentioned, liquidity and easy risk management.
That the markets provide those behaviors is what makes them valuable but the actual trades that make up those aggregates, your selling of shares when you need a car to someone else is zero sum. Either you would make more by holding or you wouldn't.
That something other than that is more important to you indicates that you are in the market for a middle man to bridge that time gap and buy some of the risk from you. Your time horizon is from share purchase to "need money for car", not from share purchase to "optimal selling point". The service you take advantage of when you bridge that gap is provided by the aggregate work of many zero sum interactions between speculative participants like market makers and "investors" like your self.
If I put a sell order on the market at 12:00 the only impact is the sales price. If it executes at 1PM or 2PM it makes zero difference to me as I can only access money at the end of the day. So, I only gain liquidity if I would have been otherwise unable to sell by the end of the day. Therefore, I don't gain liquidity from HFT.
Even stock to stock transitions can be meaningful as Bill Gates had a lot of MS stock and wanted a hedge so he sold stock. What he got was probably worth 'less' the diversification was valuable to him making the transaction a net positive.