Why do I need to warm a french press before using it? It has been recommended in a number of youtube videos and blogs to warm the frenchpress glass before brewing coffee.

Why does one need to do this?
To keep the brewing temperature as stable as possible.

When using glassware during brewing as in French-press, V60 or Chemex, it is important to warm up the gadgets prior to brewing. Normally, the water temperature is set just below boiling point (around 95 degrees Celsius) for a good brew. When you start with cold glassware, your nicely prepared close-to-boiling-point water may cool down a few degrees, which is crucial in the taste of the final cup of coffee.

Additional note: Moreover, after brewing, pouring the prepared coffee to a cold cup may decrease the temperature a few additional degrees, which renders your beverage at most warm, instead of hot.  This is mostly visible in espresso as its volume is relatively small. As a result, it is also recommended to keep the cups warmed. Some home espresso machines have additional heating trays on top to keep the cups warm before pulling an espresso shot.