A finished project gives tomorrow something stronger to build upon.

A finished project becomes a foundation. It can be shown, built on, referenced, and reused, and it changes what is available to you next. Unfinished work is not a foundation for anything, since nothing can rest on it, and it also takes up space while pretending to be an asset.

Clear one thing to completion before starting the next. What you can build afterward depends heavily on what is actually finished behind you.