IIRC SQLite originated as a Tcl extension. In Tcl at the user level "everything is a string" or a number. So it's logical SQLite would accept values as a string or number. Interestingly a Tcl function defines its own semantics, an input value means whatever the function says it means, perhaps a timestamp. SQLite inherited these attributes, and as many commenters observe, SQLite largely continues to work that way.

Implies documentation is crucial. Fortunately SQLite's documentation is among the best out there.