It’s a common practice as it relates to speculative investments that have seen significant gains but you’re unsure of future value. Selling half locks in some gains (de-risking) while still preserving possible future upside (the half you don’t sell).

When I worked in markets I would often get asked questions in the form of: should I short X? Should I trade X currency now or later?

More specifically many questioned whether they should buy GBP/USD the day before brexit or the day after. The only good advice is to buy 50% the day before and 50% the day after, that way you are wrong both times.