Hold for a while in exchanges? No dear that's not advisable, you buy and transfer immediately to a safe wallet, you're not trading bht intentions of buying is to hold for long term so what's the neee of holding for a while in the exchange? You can only hold there if your intentions is to trade with it.
People have their different reasons for choosing Bitcoin which is not basically about profits, moreover you don't expect what's not guaranteed but invest and believe that the investment would bring good result in the future. Infact profit shouldn't come first or in between bht last which is after years of accumulation.
It might not look like something that's realistic to you but that doesn't mean that there aren't others who are doing it, you need to know that moving bitcoin around costs quite a lot and for some people that transaction fee is too much for them to be paying on a weekly basis if they are buying bitcoin weekly.
In a situation like this one, the investor can decide to move their bitcoin to a secured wallet once a month, but if they still buy weekly then their assets will have to sit somewhere for around that time period before they can be moved to a secure wallet and using an exchange is the most likely, they are buying the bitcoin from those same exchanges after all.
You shouldn't just discredit someone elses ideas, they are trying to cut down on how much money they lose when then move their bitcoin and if delaying moving their bitcoin to a secured wallet is the best way for them to do this then they will do it.
Not all investor can afford to move their bought Bitcoins in regular basis. The transaction fees adds up the cost and repeatedly paying fees especially for those small investors does not make any sense.
In cases like this holding for awhile on exchange for few weeks or once their target hit is practical option they could to to reduce fees that they are paying. This is fine as long as they aim to transfer their bought funds in their self custody wallet once they hit their threshold. Majority of those or we are buying on exchange to have Bitcoin, so that action they have done is really fine.
Each investor have different approach, strategy and intended budget. So with the important part to consider is their aim or intention to hold Bitcoin for long term.