Why do I need to multiply unix timestamps by 1000 in JavaScript?

Because Javascript uses milliseconds internally, while normal UNIX timestamps are usually in seconds.

Javascript uses the number of milliseconds since epoch. Unix timestamp is seconds since epoch.

Hence, the need to convert Unix timestamp into millseconds before using it in Javascript

Unix time is the number of seconds since the epoch (1 Jan 1970). In Javascript, the Date object expects the number of milliseconds since the epoch, hence the 1000-fold difference.