As others have pointed out below, the value of the company may change at a fast rate, however that is far from the only reason that you might see many deals in

As others have pointed out below, the value of the company may change at a fast rate, however that is far from the only reason that you might see many deals in a short period. One of the primary benefits that market makers (which are often HFT firms) provide is the efficient transfer of risk. That is to say, the value of the company may not change thousands of times a second, but the willingness of existing holders of that stock to continue to do so may change rapidly. Or similarly some participant may suddenly need a hedge and buy this stock because it has the correlation their looking for. This is all to say that there are valid reasons for buying and selling a stock that have no direct (but some indirect) relationship with the value of that company. This is not a game played by retail investors, but it's not just played by HFT firms either. Institutional investors (e.g., my 401k money) will also use financial products in this way. Or put a completely different way, I may sell my Apple shares because I need the cash back to help pay for a downpayment on my house. Did I sell those shares because I think the value of the company has suddenly changed? No. There are a myriad of reasons participants may move in and out of a position.